diff --git a/umi_tools/dedup.py b/umi_tools/dedup.py
index bb48fd1e..cd3fb78f 100644
--- a/umi_tools/dedup.py
+++ b/umi_tools/dedup.py
@@ -23,7 +23,10 @@
 1. The read with the lowest number of mapping coordinates (see
 ``--multimapping-detection-method`` option)
 
-2. The read with the highest mapping quality
+2. The read with the highest mapping quality. Note that this is not
+the read sequencing quality and that if two reads have the same
+mapping quality then one will be picked at random regardless of the
+read quality.
 
 Otherwise a read is chosen at random.