OBJECTIVE To assess and compare the disease severity, the treatment properties and the frequency of motor complications in the patients with Parkinson's disease (PD) having and not having REM sleep behavior disorder (RBD). PATIENTS AND METHODS Based on chart review, patients with Parkinson's disease whose bed partners have reported prominent motor activity while dreaming were identified. Standard questionnaires assessing the presence of RBD have been addressed to these patients and their informants. Obtained data fulfilled clinical diagnostic criteria of probable RBD in 35 patients (RBD group) with the mean age at symptom onset was 61.8 years. Of them 77% were men. Clinical features of these patients concerning Hoehn-Yahr stage of PD, the severity of PD according to the Unified Parkinson's disease rating scale (UPDRS), the mean dose and duration of levodopa (LD) therapy, the presence of motor complications were compared with those of gender and age at PD-onset matched 35 PD patients without RBD (NRBD group). RESULTS The mean values of PD duration, Hoehn-Yahr stage and UPDRS scores did not differ between groups. The duration of LD therapy was significantly longer in RBD group in comparison to NRBD group (6.2 years versus 3.05 years, respectively, P<0.005) and also mean actual dose of LD was higher (460.3 mg/day versus 320.3 mg/day respectively, P<0.02). The dose and duration of dopamine agonists did not differ between groups. In RBD group, wearing-off phenomenon was significantly common (P<0.01), its duration was longer (P<0.005), and LD-related dyskinesias were more frequent (P<0.01). CONCLUSION In the current study, when compared with NRBD group, the patients with RBD required higher doses of LD treatment at an earlier stage of PD which eventually led to motor complications. In these patients, dopaminergic treatment restored UPDRS scores, but did not prevent the occurrence of RBD.