The possibility that a technical error may occur during nerve conduction studies due to ulnar nerve dislocation when the elbow is flexed has recently been suggested. We investigated normal volunteers using ultrasonography to observe the effects of ulnar nerve dislocation during elbow flexion on short-segment nerve conduction studies. We found significant conduction block in all of the subjects with ulnar nerve dislocation, and the finding was defined as a technical error caused by volume conduction. The results of the present study suggest that caution should be exercised when interpreting the results of short-segment nerve conduction studies at the across-elbow segment due to the possibility of technical error induced by ulnar nerve dislocation. GLOSSARY NCS ϭ nerve conduction study; S-NCS ϭ short-segment NCS; UNE ϭ ulnar neuropathy at the elbow. Neurology ® 2008;70:e9-e13