Thalidomide is effective in the treatment of such disabling dermatologic diseases as aphthosis, discoid lupus erythematosus, and prurigo nodularis, in which other drugs fail. However, its use can induce neuropathy necessitating caution in its administration. It was found in this electrophysiologic study of 13 patients that the data best revealing neuropathy, even when clinical abnormalities were not apparent, were reduction of sensory nerve action potential amplitude on the sural nerve, increase of somatosensory evoked potential latency following sural nerve stimulation, and reduction of sensory action potential amplitude on stimulating the median nerve at the wrist. In two patients, electrophysiologic abnormalities had increased after withdrawal, suggesting a prolonged action of thalidomide. Timely reduction of dosage, after detection of changes indicating the onset of side effects, could reduce the risk of the sometimes rapid emergence of clinical symptoms.