diff --git a/src/Sprache/Parse.cs b/src/Sprache/Parse.cs
index d5761fa..c36cfff 100644
--- a/src/Sprache/Parse.cs
+++ b/src/Sprache/Parse.cs
@@ -13,6 +13,11 @@ namespace Sprache
     /// </summary>
     public static partial class Parse
     {
+        /// <summary>
+        /// Message for a failure result when left recursion is detected.
+        /// </summary>
+        public const string LeftRecursionErrorMessage = "Left recursion in the grammar.";
+
         /// <summary>
         /// TryParse a single character matching 'predicate'
         /// </summary>
@@ -393,14 +398,14 @@ public static Parser<T> Ref<T>(Func<Parser<T>> reference)
                            if (i.Memos.ContainsKey(p))
                            {
                                var pResult = (IResult<T>)i.Memos[p];
-                               if (pResult.WasSuccessful)
+                               if (pResult.WasSuccessful) 
                                    return pResult;
-                               throw new ParseException(pResult.ToString());
+
+                               if (!pResult.WasSuccessful && pResult.Message == LeftRecursionErrorMessage)
+                                   throw new ParseException(pResult.ToString());
                            }
 
-                           i.Memos[p] = Result.Failure<T>(i,
-                               "Left recursion in the grammar.",
-                               new string[0]);
+                           i.Memos[p] = Result.Failure<T>(i, LeftRecursionErrorMessage, new string[0]);
                            var result = p(i);
                            i.Memos[p] = result;
                            return result;
diff --git a/test/Sprache.Tests/Parse.Ref.Tests.cs b/test/Sprache.Tests/Parse.Ref.Tests.cs
new file mode 100644
index 0000000..533fbaa
--- /dev/null
+++ b/test/Sprache.Tests/Parse.Ref.Tests.cs
@@ -0,0 +1,110 @@
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+
+namespace Sprache.Tests
+{
+    public class ParseRefTests
+    {
+        private static Parser<string> ParserHash = Parse.String("#").Text().Named("something");
+
+        private static Parser<string> ParserIdentifier = (from id in Parse.String("id").Text() select id).Named("identifier");
+
+        private static Parser<string> Parser1UnderTest =
+            (from _0 in Parse.Ref(() => ParserIdentifier)
+             from _1 in Parse.Ref(() => ParserHash)
+             from _2 in Parse.String("_")
+             select "alternative_1")
+            .Or(from _0 in Parse.Ref(() => ParserIdentifier)
+                from _1 in Parse.Ref(() => ParserHash)
+                select "alternative_2")
+            .Or(from _0 in ParserIdentifier select _0);
+
+        private static Parser<string> Parser2UnderTest = 
+            (from _0 in Parse.String("a").Text()
+             from _1 in Parse.Ref(() => Parser2UnderTest)
+             select _0 + _1)
+            .Or(from _0 in Parse.String("b").Text()
+                from _1 in Parse.Ref(() => Parser2UnderTest)
+                select _0 + _1)
+            .Or(from _0 in Parse.String("0").Text() select _0);
+
+        private static Parser<string> Parser3UnderTest =
+            (from _0 in Parse.Ref(() => Parser3UnderTest)
+             from _1 in Parse.String("a").Text()
+             select _0 + _1)
+            .Or(from _0 in Parse.String("b").Text()
+                from _1 in Parse.Ref(() => Parser3UnderTest)
+                select _0 + _1)
+            .Or(from _0 in Parse.String("0").Text() select _0);
+
+        private static Parser<string> Parser4UnderTest =
+            from _0 in Parse.Ref(() => Parser4UnderTest)
+            select "simplest left recursion";
+
+        private static Parser<string> Parser5UnderTest =
+          (from _0 in Parse.String("_").Text()
+           from _1 in Parse.Ref(() => Parser5UnderTest)
+           select _0 + _1)
+          .Or(from _0 in Parse.String("+").Text()
+              from _1 in Parse.Ref(() => Parser5UnderTest)
+              select _0 + _1)
+          .Or(Parse.Return(""));
+
+        [Fact]
+        public void MultipleRefs() => AssertParser.SucceedsWith(Parser1UnderTest, "id=1", o => Assert.Equal("id", o));
+
+        [Fact]
+        public void RecursiveParserWithoutLeftRecursion() => AssertParser.SucceedsWith(Parser2UnderTest, "ababba0", o => Assert.Equal("ababba0", o));
+
+        [Fact]
+        public void RecursiveParserWithLeftRecursion() => Assert.Throws<ParseException>(() => Parser3UnderTest.TryParse("b0"));
+
+        [Fact]
+        public void SimplestLeftRecursion() => Assert.Throws<ParseException>(() => Parser4UnderTest.TryParse("test"));
+
+        [Fact]
+        public void EmptyAlternative1() => AssertParser.SucceedsWith(Parser5UnderTest, "_+_+a", o => Assert.Equal("_+_+", o));
+
+        [Fact]
+        public void Issue166()
+        {
+            var letterA = Parse.Char('a');
+            var letterReferenced = Parse.Ref(() => letterA);
+            var someAlternative = letterReferenced.Or(letterReferenced);
+
+            Assert.False(someAlternative.TryParse("b").WasSuccessful);
+        }
+
+        static readonly Parser<IEnumerable<char>> ASeq =
+            (from first in Parse.Ref(() => ASeq)
+             from comma in Parse.Char(',')
+             from rest in Parse.Char('a').Once()
+             select first.Concat(rest))
+            .Or(Parse.Char('a').Once());
+
+        [Fact]
+        public void DetectsLeftRecursion()
+        {
+            Assert.Throws<ParseException>(() => ASeq.TryParse("a,a,a"));
+        }
+
+        static readonly Parser<IEnumerable<char>> ABSeq =
+            (from first in Parse.Ref(() => BASeq)
+             from rest in Parse.Char('a').Once()
+             select first.Concat(rest))
+            .Or(Parse.Char('a').Once());
+
+        static readonly Parser<IEnumerable<char>> BASeq =
+            (from first in Parse.Ref(() => ABSeq)
+             from rest in Parse.Char('b').Once()
+             select first.Concat(rest))
+            .Or(Parse.Char('b').Once());
+
+        [Fact]
+        public void DetectsMutualLeftRecursion()
+        {
+            Assert.Throws<ParseException>(() => ABSeq.End().TryParse("baba"));
+        }
+    }
+}
diff --git a/test/Sprache.Tests/ParseTests.cs b/test/Sprache.Tests/ParseTests.cs
index 01108a1..5336146 100644
--- a/test/Sprache.Tests/ParseTests.cs
+++ b/test/Sprache.Tests/ParseTests.cs
@@ -144,37 +144,6 @@ public void ParsesString_AsSequenceOfChars()
             AssertParser.SucceedsWithAll(p, "abc");
         }
 
-        static readonly Parser<IEnumerable<char>> ASeq =
-            (from first in Parse.Ref(() => ASeq)
-             from comma in Parse.Char(',')
-             from rest in Parse.Char('a').Once()
-             select first.Concat(rest))
-            .Or(Parse.Char('a').Once());
-
-        [Fact]
-        public void DetectsLeftRecursion()
-        {
-            Assert.Throws<ParseException>(() => ASeq.TryParse("a,a,a"));
-        }
-
-        static readonly Parser<IEnumerable<char>> ABSeq =
-            (from first in Parse.Ref(() => BASeq)
-             from rest in Parse.Char('a').Once()
-             select first.Concat(rest))
-            .Or(Parse.Char('a').Once());
-
-        static readonly Parser<IEnumerable<char>> BASeq =
-            (from first in Parse.Ref(() => ABSeq)
-             from rest in Parse.Char('b').Once()
-             select first.Concat(rest))
-            .Or(Parse.Char('b').Once());
-
-        [Fact]
-        public void DetectsMutualLeftRecursion()
-        {
-            Assert.Throws<ParseException>(() => ABSeq.End().TryParse("baba"));
-        }
-
         [Fact]
         public void WithMany_WhenLastElementFails_FailureReportedAtLastElement()
         {