To determine the extent to which depression complicates Parkinson's disease (PD), the authors analyzed the literature on depression in PD in order to report on its prevalence, clinical manifestations, and treatment. By means of MEDLINE literature searches, the analysis focused on 45 PD depression studies conducted from 1922 through 1998. The results indicate that the prevalence of depression is 31% for all PD patients. The clinical manifestations of PD depression include apathy, psychomotor retardation, memory impairment, pessimism, irrationality, and suicidal ideation without suicidal behavior. PD depression is effectively treated with a variety of antidepressants, most commonly at present the selective serotonin reuptake inhibitors. Anecdotal evidence supports the use of sertraline to treat PD depression.