OBJECTIVE: To explore the diagnostic value of the nerve conduction study in the dorsal nerve of penis before and after erection. METHOD: Nineteen subjects with a confirmed psychogenic impotence were studied for the superficial peroneal and sural sensory nerve conductions, bulbocavernous reflexes (BCR), tibial and pudendal somatosensory evoked potentials (SEP), and conduction studies of the dorsal nerve of penis. Sixteen subjects with normal findings had repeat studies after an injection of Prostaglandin E1. RESULTS: Erection helped to obtain a dorsal penile sensory potentials in 18.8% of the 16 subjects. The mean conduction velocity of the dorsal nerve of penis increased significantly from 33.8+/-9.3 m/sec to 48.4+/-12.8 m/sec. The mean sensory amplitude of the dorsal nerve of penis increased significantly from 1.7+/-1.0 microvolt to 2.8+/-1.4 microvolt. There was no significant difference in the BCR latency and the pudendal SEP (latency and amplitude) before and after erection. CONCLUSION: Erection helps to obtain the sensory potentials of the dorsal nerve of penis when it is technically difficult to obtain and increases the conduction velocity as well as amplitude of the dorsal nerve of penis. However it does not affect the BCR latency and pudendal SEP.