Twenty of 23 patients with chronic obstructive pulmonary disease (COPD) showed electrophysiologic evidence of peripheral nerve dysfunction. Abnormalities of sensory nerve conduction were most common, affecting the sural nerve (20 subjects), ulnar nerve (11), radial nerve (eight), and median nerve (seven). Six subjects had impairment of both sensory and motor nerve function, with the common peroneal being the most frequently affected motor nerve. Clinical signs of neuropathy were found in four patients. Cigarette smoking, expressed as pack-years, was correlated significantly with the electrophysiologic abnormalities. These findings indicate that subclinical polyneuropathy commonly occurs in association with COPD and that this COPD-related neuropathy is correlated with cigarette consumption. From these data we suggest that a substance or substances in cigarette smoke, such as nicotine, taken on a long-term basis, may be toxic to peripheral nerves.