A consecutive series of patients with Parkinson's disease (PD) were examined for the presence of sleep disturbances, pain, and depression. We found that patients with PD and major depression had significantly more sleep disturbances and severe pain than non-depressed patients with PD. Moreover, depression scores accounted for most of the variance in a stepwise regression analysis of the effect of numerous clinical variables on either sleep disorders or pain severity. These findings suggest that depression is the most important factor associated with the common problems of sleep disorder and pain among patients with PD.