Depression is a common feature of Parkinson's disease, a fact of both clinical and theoretical significance. Assessment of depression in Parkinson's disease is complicated by overlapping symptomatology in the two conditions, making global assessments based on observer or self-ratings of doubtful validity. The present study aimed to provide both a quantitative and qualitative description of the nature of the depressive changes found in Parkinson's disease as compared with normal elderly subjects and arthritis patients. As with previous studies, the patients with Parkinson's disease scored significantly higher than normal controls on various self-ratings of depression and anxiety but, in this study, did not differ from those with arthritis. Qualitatively, both the Parkinson's disease and the arthritis groups had depression characterised by pessimism and hopelessness, decreased motivation and drive, and increased concern with health. In contrast, the negative affective feelings of guilt, self-blame and worthlessness were absent in both patient groups. This pattern of depression was significantly associated with severity of illness and functional disability. However, these factors account for only a modest proportion of the variability in test scores. Probable unexplored factors are individual differences in coping style and availability of support.