The relative benefits of selegeline hydrochloride (2 x 5 mg, 2 x 10 mg selegeline) were studied in 30 narcoleptic patients using a randomized, double-blind, placebo-controlled design. Patients were randomly assigned to three groups (placebo and 2 x 5 mg and 2 x 10 mg selegeline). After a 2-week washout period for previous anticataplectic and stimulant medication, the study started with a 2-day period of placebo intake for each group, continued by 14 days of medication, ending with a 2-day placebo period. Outcome was measured by comparison of four polysomnographies and four multiple sleep latency tests (MSLTs) performed during the initial and the final placebo and medication period. Each MSLT day included acoustic and visual vigilance tests. Blood pressure and pulse rate were monitored daily. Patients reported daily about mood, concentration, subjective sleep time, nocturnal awakenings, nocturnal wake times, number of naps, and occurrence of symptoms of the narcoleptic tetrad. Selegeline caused dose-dependent REM suppression during nighttime sleep and naps and increase of sleep and REM latency. Under selegeline, daytime sleepiness improved significantly and the number of sleep attacks and naps as well as the frequency of cataplexy were reduced. Selegeline at a dose of at least 20 mg/day is a potent drug for the treatment of all narcoleptic symptoms.