We performed a clinical and electrophysiologic study of median and ulnar nerve function to determine the frequency of neuropathy in 21 patients who had unilateral Brescia-Cimino arteriovenous fistulas and who were undergoing maintenance hemodialysis. Seven patients had symptomatic median or ulnar neuropathy in the arm with the fistula, and abnormalities of motor and/or sensory nerve conduction were found in all of these patients. Of the 14 asymptomatic patients, nine had electrophysiologic evidence of median and/or ulnar neuropathy in the arm with the fistula. Evidence of subclinical median or ulnar neuropathy was also found in the contralateral extremity in 11 of the 21 subjects. Statistically significant differences were found for median and ulnar sensory nerve action potential latencies and motor conduction velocities and for the median distal motor latency between the arms with and without fistulas in the group as a whole, and the mean interarm differences for these values were statistically significant.