Six patients with restless legs syndrome (RLS) and periodic movements during sleep (PMS) received placebo or L-dopa in a double-blind study. We recorded patients for 36 consecutive hours in the sleep laboratory during a baseline investigation and at the end of each treatment period. Daily evening questionnaires and a suggested immobilization test (SIT) performed at bedtime on each recording night documented the effect of L-dopa in RLS. A nocturnal EMG recording of the anterior tibialis muscles revealed the effect of L-dopa on PMS. L-Dopa proved effective in treating both RLS and PMS. Although not present in every patient, leg movements recorded during the SIT exhibited a clear periodicity. These observations support the hypothesis that RLS and PMS are two manifestations of the same central sensorimotor disorder.