diff --git a/CHANGES.txt b/CHANGES.txt
index 8248bbd96..0b5aad921 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -10,6 +10,7 @@ November 18, 2012
 ** labels in lexer rules
 ** tokens {A;B;} syntax
 ** tokens {A='C';} syntax
+** {...}?=> gate semantic predicates
 
 November 17, 2012
 
diff --git a/tool/src/org/antlr/v4/Tool.java b/tool/src/org/antlr/v4/Tool.java
index 82df03793..e39b22d1c 100644
--- a/tool/src/org/antlr/v4/Tool.java
+++ b/tool/src/org/antlr/v4/Tool.java
@@ -41,10 +41,10 @@ import org.antlr.v4.automata.LexerATNFactory;
 import org.antlr.v4.automata.ParserATNFactory;
 import org.antlr.v4.codegen.CodeGenPipeline;
 import org.antlr.v4.misc.Graph;
-import org.antlr.v4.parse.ANTLRLexer;
 import org.antlr.v4.parse.ANTLRParser;
 import org.antlr.v4.parse.GrammarASTAdaptor;
 import org.antlr.v4.parse.GrammarTreeVisitor;
+import org.antlr.v4.parse.ToolANTLRLexer;
 import org.antlr.v4.parse.ToolANTLRParser;
 import org.antlr.v4.parse.v3TreeGrammarException;
 import org.antlr.v4.runtime.misc.LogManager;
@@ -600,7 +600,7 @@ public class Tool {
 	public GrammarRootAST load(String fileName, CharStream in) {
 		try {
 			GrammarASTAdaptor adaptor = new GrammarASTAdaptor(in);
-			ANTLRLexer lexer = new ANTLRLexer(in);
+			ToolANTLRLexer lexer = new ToolANTLRLexer(in, this);
 			CommonTokenStream tokens = new CommonTokenStream(lexer);
 			lexer.tokens = tokens;
 			ToolANTLRParser p = new ToolANTLRParser(tokens, this);
diff --git a/tool/src/org/antlr/v4/parse/ANTLRLexer.g b/tool/src/org/antlr/v4/parse/ANTLRLexer.g
index 170dab432..dd7b8d5b1 100644
--- a/tool/src/org/antlr/v4/parse/ANTLRLexer.g
+++ b/tool/src/org/antlr/v4/parse/ANTLRLexer.g
@@ -297,7 +297,17 @@ ARG_ACTION
 // the delimiting {} is no additional overhead.
 //
 ACTION
-	:	NESTED_ACTION ('?' {$type = SEMPRED;} )?
+	:	NESTED_ACTION
+		('?' {$type = SEMPRED;} )?
+		(	'=>' // v3 gated sempred
+			{
+			Token t = new CommonToken(input, state.type, state.channel, state.tokenStartCharIndex, getCharIndex()-1);
+			t.setLine(state.tokenStartLine);
+			t.setText(state.text);
+			t.setCharPositionInLine(state.tokenStartCharPositionInLine);
+            grammarError(ErrorType.V3_GATED_SEMPRED, t);
+			}
+		)?
 	;
 
 // ----------------
diff --git a/tool/src/org/antlr/v4/parse/ToolANTLRLexer.java b/tool/src/org/antlr/v4/parse/ToolANTLRLexer.java
new file mode 100644
index 000000000..9f84ddc53
--- /dev/null
+++ b/tool/src/org/antlr/v4/parse/ToolANTLRLexer.java
@@ -0,0 +1,20 @@
+package org.antlr.v4.parse;
+
+import org.antlr.runtime.CharStream;
+import org.antlr.runtime.Token;
+import org.antlr.v4.Tool;
+import org.antlr.v4.tool.ErrorType;
+
+public class ToolANTLRLexer extends ANTLRLexer {
+	public Tool tool;
+
+	public ToolANTLRLexer(CharStream input, Tool tool) {
+		super(input);
+		this.tool = tool;
+	}
+
+	@Override
+	public void grammarError(ErrorType etype, Token token, Object... args) {
+		tool.errMgr.grammarError(etype, getSourceName(), token, args);
+	}
+}
diff --git a/tool/src/org/antlr/v4/tool/ErrorType.java b/tool/src/org/antlr/v4/tool/ErrorType.java
index 1adcb4d53..a42caf713 100644
--- a/tool/src/org/antlr/v4/tool/ErrorType.java
+++ b/tool/src/org/antlr/v4/tool/ErrorType.java
@@ -123,6 +123,7 @@ public enum ErrorType {
 		"getText() to get the entire text matched for the rule", ErrorSeverity.WARNING_ONE_OFF),
 	V3_TOKENS_SYNTAX(202, "'tokens {A; B;}' syntax is now 'tokens {A, B}' in ANTLR v4", ErrorSeverity.WARNING),
 	V3_ASSIGN_IN_TOKENS(203, "assignments in tokens{} are not supported in ANTLR v4; use lexical rule '<arg> : <arg2>;' instead", ErrorSeverity.ERROR_ONE_OFF),
+	V3_GATED_SEMPRED(204, "{...}?=> gate semantic predicates in v3 behave like normal predicates in ANTLR v4; use {...}? instead", ErrorSeverity.WARNING_ONE_OFF),
 
     // Dependency sorting errors