Depression and nocturnal disturbances are frequent in patients with Parkinson's disease (PD). The aim of this study was to determine the correlation between depressive symptoms and nocturnal disturbances in patients with PD in Japan. The subjects of this multi-center cross-sectional study were 188 patients with PD and 144 age-matched controls who were assessed for nocturnal disturbances by the Parkinson's disease sleep scale (PDSS) and for depressive symptoms by Zung Self-Rating Depression Scale (SDS). Depressive symptoms (SDS score of > or =40) were identified in 122 patients (64.9%). The SDS was significantly higher in PD patients than control subjects. The stepwise regression model identified PDSS (p<0.001) and Unified Parkinson's Disease Rating Scale I (mental state) (p=0.002) as significant determinants of SDS. Stepwise regression analysis identified item 15 (daytime sleepiness) (p=0.002), item 13 (early morning tremor) (p=0.008), item 12 (nocturnal dystonia) (p=0.015), and item 3 (sleep maintenance insomnia) (p=0.026) as significant predictors of SDS. Our results indicated that depressive symptoms in PD correlate significantly with nocturnal disturbances, and that daytime sleepiness, dystonia, tremor and sleep fragmentation are the most common nocturnal disturbances in depressed patients with PD.