diff --git a/util/relation/src/main/scala/sbt/Relation.scala b/util/relation/src/main/scala/sbt/Relation.scala
index d97ee2321..725512d0b 100644
--- a/util/relation/src/main/scala/sbt/Relation.scala
+++ b/util/relation/src/main/scala/sbt/Relation.scala
@@ -118,7 +118,7 @@ private final class MRelation[A,B](fwd: Map[A, Set[B]], rev: Map[B, Set[A]]) ext
 	def _1s = fwd.keySet
 	def _2s = rev.keySet
 
-	def size = fwd.size
+	def size = (fwd.valuesIterator map { _.size }).foldLeft(0)(_ + _)
 	
 	def all: Traversable[(A,B)] = fwd.iterator.flatMap { case (a, bs) => bs.iterator.map( b => (a,b) ) }.toTraversable
 
diff --git a/util/relation/src/test/scala/RelationTest.scala b/util/relation/src/test/scala/RelationTest.scala
index 03b728915..3dcc03f38 100644
--- a/util/relation/src/test/scala/RelationTest.scala
+++ b/util/relation/src/test/scala/RelationTest.scala
@@ -60,6 +60,13 @@ object RelationTest extends Properties("Relation")
 		}
 	}
 
+  property("Computes size correctly") = forAll { (entries: List[(Int, Double)]) =>
+    val rel = Relation.empty[Int, Double] ++ entries
+    val expected = rel.all.size  // Note: not entries.length, as entries may have duplicates.
+    val computed = rel.size
+    "Expected size: %d. Computed size: %d.".format(expected, computed) |: expected == computed
+  }
+
 	def all[T](s: Seq[T])(p: T => Prop): Prop =
 		if(s.isEmpty) true else s.map(p).reduceLeft(_ && _)
 }