Periodic leg movements during sleep (PMS) is a disorder frequently encountered in narcolepsy. In the present study, 12 narcoleptic patients (six with PMS and six without) were recorded in a sleep laboratory for 2 consecutive nights before and after treatment with gamma-hydroxybutyrate (GHB) taken at bedtime for 1 month. Treatment resulted in decreased rapid eye movement (REM) sleep latency and increased REM efficiency without change in the total duration of REM sleep. GHB was associated with the appearance of pathological levels of PMS in patients who were unaffected before treatment. These results are discussed in relation to the role of dopamine in the physiopathology of narcolepsy and PMS.