Simple, relatively quick, electrophysiologic tests make it possible to diagnose the early cubital tunnel syndrome. Electrophysiologic investigation demonstrated dysfunction of the ulnar nerve due to a lesion localized to the elbow in (1) 60 percent of 56 patients with a presumptive diagnosis of cubital tunnel syndrome but no confirmatory clinical signs, (2) 25 percent of 48 control subjects, and (3) 91 percent of 34 patients with clear-cut clinical evidence of an ulnar nerve compression at the elbow. The terminal motor latency from above the elbow to the abductor digiti quinti minimi muscle was of the most value in diagnosing early cubital tunnel syndrome. If this latency is greater than 9 msec, the diagnosis of the syndrome should be seriously considered.