Parkinson's disease (PD) is frequently accompanied by symptoms of depression and anxiety. However, the relationship between anxiety and depression has not been rigorously defined in these patients. In this study, 42 patients with PD and 21 matched medical controls were evaluated using DSM-III-R criteria and a variety of psychiatric rating scales. Twelve (29%) PD patients but only one medical control had a formal anxiety disorder diagnosis. Of the 12 patients with PD who had an anxiety disorder diagnosis, 11 (92%) had a comorbid depressive disorder diagnosis. Of the 18 patients with a depressive disorder, 12 (67%) also had an anxiety disorder diagnosis. Furthermore, a stepwise regression analysis found that the depression measure explained 44% of the variance in anxiety measures whereas neither the severity of illness variables nor the levodopa dose contributed significantly to the variance. This study suggests that the excess anxiety found in PD patients is unlikely to be primarily a psychologic reaction to the illness or a side effect of levodopa treatment. Rather, we suggest that anxiety and depression are related manifestations of the underlying neurochemical changes of PD itself.