Penile sympathic skin response (PSSR) was measured in 20 normal patients and in 46 patients with erectile dysfunction (ED). PSSR could be registered in 80% of normal patients. The latencies were 1,100-1,600 ms. In patients with ED, latencies were 1,240-3,640 ms. A reproducible normal latency of PSSR (< 1,600 ms) indicates a normal sympathic innervation of the penis. Long latencies (> 1,800 ms) indicate neuropathy of unmyelinated nerval fibers. The loss of a reproducible answer however allows no exact diagnostic information. PSSR is a useful diagnostic test for detecting neuropathy of unmyelinated nerval fibers as a possible cause of ED.