The development, clinical course, and electrophysiologic characteristics of narcolepsy were evaluated in 50 adults who had a current complaint of sleep attacks and cataplexy. In most of the patients, the first symptoms, usually excessive daytime sleepiness and sleep attacks, developed during childhood or adolescence. The condition was invariably chronic. Patients frequently had family histories of some disorder of excessive daytime sleepiness. In nocturnal sleep or daytime nap recordings, all but three of the patients demonstrated a rapid-eye-movement (REM) period at sleep onset. Sleep apnea was found in only one patient. Our findings indicate that sleep laboratory recordings to detect a sleep-on-set REM period are of little diagnostic value when the narcoleptic patient has cataplexy. Furthermore, narcoleptic patients require sleep laboratory evaluation for sleep apnea only when the presence of apnea is suggested by the sleep history.