Insomnia, sleep fragmentation and excessive daytime sleepiness are common in Parkinson's disease (PD) and may contribute to the reduction of cognition and alertness in those patients. Melatonin has been shown to improve sleep in several conditions. In experimental models of PD, melatonin can ameliorate motor symptoms. To evaluate the effect of melatonin on sleep and motor dysfuntion in PD, we studied 18 patients (Hoehn & Yahr I to III) from a PD clinic. Prior to treatment, motor dysfunction was assessed by UPDRS II, III and IV. Subjective sleep quality was assessed by the Pittsburgh Sleep Quality Index (PSQI) and daytime somnolence by the Epworth Sleepiness Scale (ESS). Full polysomnography (PSG) was performed in all subjects. Patients were then randomized to receive melatonin (3mg) or placebo one hour before bedtime for four weeks. All measures were repeated at the end of treatment. On initial assessment, 14 patients (70%) showed poor quality sleep (PSQI > 6) and eight (40%) excessive daytime sleepiness (ESS > 10). Increased sleep latency (50%), REM sleep without atonia (66%), and reduced sleep efficiency (72%) were found on PSG. Eight patients had an apnea/ hipopnea index greater than 15 but no severe oxygen desaturation was observed. Sleep fragmentation tended to be more severe in patients on lower doses of levodopa (p = 0.07). Although melatonin significantly improved subjective quality of sleep (p = 0.03) as evaluated by the PSQI index, PSG abnormalities were not changed. Motor dysfunction was not improved by the use of melatonin. Undetected differences in motor scores and PSG findings may have been due to a small sample size and a type II error.