Objective:Electrophysiological diagnosis of ulnar nerve entrapment at the wrist is sometimes difficult. The aim was to evaluate the diagnostic role of ulnar nerve stimulation above and below the Guyon channel in ulnar nerve entrapment at the wrist.Methods:Supramaximal ulnar nerve stimulation at the wrist and palm, in addition to the standard nerve conduction studies, in 10 patients with ulnar nerve entrapment at the wrist and 40 controls.Results:Motor latencies to the first dorsal interosseous muscle with wrist stimulation were prolonged unilaterally in six and bilaterally in four patients. Palmar stimulation showed partial conduction blocks on the more symptomatic side in all. In two bilateral cases, there were 28.8% and 44.3% amplitude loss on the less symptomatic side. Motor responses elicited with palm stimulation were also found to have prolonged latencies and decreased amplitudes.Conclusions:Stimulation of the deep branch of the ulnar nerve can expand the information about ulnar nerve entrapment at the wrist by providing evidence about its nature, prognosis, precise localization, and distal extent.