diff --git a/integration_tests/src/main/python/window_function_test.py b/integration_tests/src/main/python/window_function_test.py
index b9717bb61d7..e6e92909502 100644
--- a/integration_tests/src/main/python/window_function_test.py
+++ b/integration_tests/src/main/python/window_function_test.py
@@ -101,7 +101,6 @@ def tmp(something):
         return meta + idfn(something)
     return tmp
 
-@pytest.mark.xfail(condition=not(is_before_spark_310()), reason='https://github.com/NVIDIA/spark-rapids/issues/999')
 @ignore_order
 @approximate_float
 @pytest.mark.parametrize('c_gen', lead_lag_data_gens, ids=idfn)
diff --git a/shims/spark311/src/main/scala/com/nvidia/spark/rapids/shims/spark311/OffsetWindowFunctionMeta.scala b/shims/spark311/src/main/scala/com/nvidia/spark/rapids/shims/spark311/OffsetWindowFunctionMeta.scala
new file mode 100644
index 00000000000..613630fdcc8
--- /dev/null
+++ b/shims/spark311/src/main/scala/com/nvidia/spark/rapids/shims/spark311/OffsetWindowFunctionMeta.scala
@@ -0,0 +1,82 @@
+/*
+ * Copyright (c) 2021, NVIDIA CORPORATION.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.nvidia.spark.rapids.shims.spark311
+
+import com.nvidia.spark.rapids.{BaseExprMeta, DataFromReplacementRule, ExprMeta, GpuOverrides, RapidsConf, RapidsMeta}
+
+import org.apache.spark.sql.catalyst.expressions.{Expression, Lag, Lead, Literal, OffsetWindowFunction}
+import org.apache.spark.sql.types.IntegerType
+
+/**
+ * Spark 3.1.1-specific replacement for com.nvidia.spark.rapids.OffsetWindowFunctionMeta.
+ * This is required primarily for two reasons:
+ *   1. com.nvidia.spark.rapids.OffsetWindowFunctionMeta (compiled against Spark 3.0.x)
+ *      fails class load in Spark 3.1.x. (`expr.input` is not recognized as an Expression.)
+ *   2. The semantics of offsets in LAG() are reversed/negated in Spark 3.1.1.
+ *      E.g. The expression `LAG(col, 5)` causes Lag.offset to be set to `-5`,
+ *      as opposed to `5`, in prior versions of Spark.
+ * This class adjusts the LAG offset to use similar semantics to Spark 3.0.x.
+ */
+abstract class OffsetWindowFunctionMeta[INPUT <: OffsetWindowFunction] (
+    expr: INPUT,
+    conf: RapidsConf,
+    parent: Option[RapidsMeta[_, _, _]],
+    rule: DataFromReplacementRule)
+  extends ExprMeta[INPUT](expr, conf, parent, rule) {
+  lazy val input: BaseExprMeta[_] = GpuOverrides.wrapExpr(expr.input, conf, Some(this))
+  lazy val adjustedOffset: Expression = {
+    expr match {
+      case lag: Lag =>
+        GpuOverrides.extractLit(lag.offset) match {
+         case Some(Literal(offset: Int, IntegerType)) =>
+            Literal(-offset, IntegerType)
+         case _ =>
+           throw new IllegalStateException(
+             s"Only integer literal offsets are supported for LAG. Found:${lag.offset}")
+        }
+      case lead: Lead =>
+        GpuOverrides.extractLit(lead.offset) match {
+          case Some(Literal(offset: Int, IntegerType)) =>
+            Literal(offset, IntegerType)
+          case _ =>
+            throw new IllegalStateException(
+              s"Only integer literal offsets are supported for LEAD. Found:${lead.offset}")
+        }
+      case other =>
+        throw new IllegalStateException(s"$other is not a supported window function")
+    }
+  }
+  lazy val offset: BaseExprMeta[_] =
+    GpuOverrides.wrapExpr(adjustedOffset, conf, Some(this))
+  lazy val default: BaseExprMeta[_] = GpuOverrides.wrapExpr(expr.default, conf, Some(this))
+
+  override val childExprs: Seq[BaseExprMeta[_]] = Seq.empty
+
+  override def tagExprForGpu(): Unit = {
+    expr match {
+      case Lead(_,_,_) => // Supported.
+      case Lag(_,_,_) =>  // Supported.
+      case other =>
+        willNotWorkOnGpu( s"Only LEAD/LAG offset window functions are supported. Found: $other")
+    }
+
+    if (GpuOverrides.extractLit(expr.offset).isEmpty) { // Not a literal offset.
+      willNotWorkOnGpu(
+        s"Only integer literal offsets are supported for LEAD/LAG. Found: ${expr.offset}")
+    }
+  }
+}
diff --git a/shims/spark311/src/main/scala/com/nvidia/spark/rapids/shims/spark311/Spark311Shims.scala b/shims/spark311/src/main/scala/com/nvidia/spark/rapids/shims/spark311/Spark311Shims.scala
index eee1772b019..4d090113eca 100644
--- a/shims/spark311/src/main/scala/com/nvidia/spark/rapids/shims/spark311/Spark311Shims.scala
+++ b/shims/spark311/src/main/scala/com/nvidia/spark/rapids/shims/spark311/Spark311Shims.scala
@@ -138,7 +138,6 @@ class Spark311Shims extends Spark301Shims {
 
         // stringChecks are the same
         // binaryChecks are the same
-
         override val decimalChecks: TypeSig = none
         override val sparkDecimalSig: TypeSig = numeric + BOOLEAN + STRING
 
@@ -196,8 +195,37 @@ class Spark311Shims extends Spark301Shims {
             childExprs(1).convertToGpu(),
             childExprs(2).convertToGpu())
         }
+      }),
+    // Spark 3.1.1-specific LEAD expression, using custom OffsetWindowFunctionMeta.
+    GpuOverrides.expr[Lead](
+      "Window function that returns N entries ahead of this one",
+      ExprChecks.windowOnly(TypeSig.numeric + TypeSig.BOOLEAN +
+        TypeSig.DATE + TypeSig.TIMESTAMP, TypeSig.all,
+        Seq(ParamCheck("input", TypeSig.numeric + TypeSig.BOOLEAN +
+          TypeSig.DATE + TypeSig.TIMESTAMP, TypeSig.all),
+          ParamCheck("offset", TypeSig.INT, TypeSig.INT),
+          ParamCheck("default", TypeSig.numeric + TypeSig.BOOLEAN +
+            TypeSig.DATE + TypeSig.TIMESTAMP + TypeSig.NULL, TypeSig.all))),
+      (lead, conf, p, r) => new OffsetWindowFunctionMeta[Lead](lead, conf, p, r) {
+        override def convertToGpu(): GpuExpression =
+          GpuLead(input.convertToGpu(), offset.convertToGpu(), default.convertToGpu())
+      }),
+    // Spark 3.1.1-specific LAG expression, using custom OffsetWindowFunctionMeta.
+    GpuOverrides.expr[Lag](
+      "Window function that returns N entries behind this one",
+      ExprChecks.windowOnly(TypeSig.numeric + TypeSig.BOOLEAN +
+        TypeSig.DATE + TypeSig.TIMESTAMP, TypeSig.all,
+        Seq(ParamCheck("input", TypeSig.numeric + TypeSig.BOOLEAN +
+          TypeSig.DATE + TypeSig.TIMESTAMP, TypeSig.all),
+          ParamCheck("offset", TypeSig.INT, TypeSig.INT),
+          ParamCheck("default", TypeSig.numeric + TypeSig.BOOLEAN +
+            TypeSig.DATE + TypeSig.TIMESTAMP + TypeSig.NULL, TypeSig.all))),
+      (lag, conf, p, r) => new OffsetWindowFunctionMeta[Lag](lag, conf, p, r) {
+        override def convertToGpu(): GpuExpression = {
+          GpuLag(input.convertToGpu(), offset.convertToGpu(), default.convertToGpu())
+        }
       })
-  ).map(r => (r.getClassFor.asSubclass(classOf[Expression]), r)).toMap
+ ).map(r => (r.getClassFor.asSubclass(classOf[Expression]), r)).toMap
 
   override def getExprs: Map[Class[_ <: Expression], ExprRule[_ <: Expression]] = {
     super.exprs301 ++ exprs311
diff --git a/sql-plugin/src/main/scala/com/nvidia/spark/rapids/GpuWindowExpression.scala b/sql-plugin/src/main/scala/com/nvidia/spark/rapids/GpuWindowExpression.scala
index 564a1c469bb..daff661f56f 100644
--- a/sql-plugin/src/main/scala/com/nvidia/spark/rapids/GpuWindowExpression.scala
+++ b/sql-plugin/src/main/scala/com/nvidia/spark/rapids/GpuWindowExpression.scala
@@ -738,10 +738,43 @@ abstract class OffsetWindowFunctionMeta[INPUT <: OffsetWindowFunction] (
     parent: Option[RapidsMeta[_, _, _]],
     rule: DataFromReplacementRule)
     extends ExprMeta[INPUT](expr, conf, parent, rule) {
-  val input: BaseExprMeta[_] = GpuOverrides.wrapExpr(expr.input, conf, Some(this))
-  val offset: BaseExprMeta[_] = GpuOverrides.wrapExpr(expr.offset, conf, Some(this))
-  val default: BaseExprMeta[_] = GpuOverrides.wrapExpr(expr.default, conf, Some(this))
-  override val childExprs: Seq[BaseExprMeta[_]] = Seq(input, offset, default)
+  lazy val input: BaseExprMeta[_] = GpuOverrides.wrapExpr(expr.input, conf, Some(this))
+  lazy val offset: BaseExprMeta[_] = {
+    expr match {
+      case Lead(_,_,_) => // Supported.
+      case Lag(_,_,_) =>  // Supported.
+      case other =>
+        throw new IllegalStateException(
+          s"Only LEAD/LAG offset window functions are supported. Found: $other")
+    }
+
+    val literalOffset = GpuOverrides.extractLit(expr.offset) match {
+      case Some(Literal(offset: Int, IntegerType)) =>
+        Literal(offset, IntegerType)
+      case _ =>
+        throw new IllegalStateException(
+          s"Only integer literal offsets are supported for LEAD/LAG. Found: ${expr.offset}")
+    }
+
+    GpuOverrides.wrapExpr(literalOffset, conf, Some(this))
+  }
+  lazy val default: BaseExprMeta[_] = GpuOverrides.wrapExpr(expr.default, conf, Some(this))
+
+  override val childExprs: Seq[BaseExprMeta[_]] = Seq.empty
+
+  override def tagExprForGpu(): Unit = {
+    expr match {
+      case Lead(_,_,_) => // Supported.
+      case Lag(_,_,_) =>  // Supported.
+      case other =>
+        willNotWorkOnGpu( s"Only LEAD/LAG offset window functions are supported. Found: $other")
+    }
+
+    if (GpuOverrides.extractLit(expr.offset).isEmpty) { // Not a literal offset.
+      willNotWorkOnGpu(
+        s"Only integer literal offsets are supported for LEAD/LAG. Found: ${expr.offset}")
+    }
+  }
 }
 
 trait GpuOffsetWindowFunction extends GpuAggregateWindowFunction {