OBJECTIVES: To assess the influence of depression, dementia and disease severity on the health-related quality of life in Parkinson's disease (PD). METHODS: Health-related quality of life was assessed in 91 patients with PD (male: 62; female: 29; mean age: 67±9) using the Parkinson's disease questionnaire (PDQ-39) and the Parkinson's disease Quality of life questionnaire (PDQL). Dementia was evaluated by using the MMSE and depression was rated with the Beck's Depression Inventory (BDI). Disease severity and motor dysfunction were recorded with the Hoehn and Yahr scale and the UPDRS II-IV. Pearson's correlation coefficient was used to calculate correlation scores.. RESULTS: Both specific disease QoL rating scales correlated with depressive symptoms (r = 0.73, p < .0001). UPDRS III (r = 0.69, p < .0001) and HY stage (r = .59, p < .001) also correlated with decreased QoL. However, motor fluctuations and dyskinesias had a minor impact on QoL (r = 0.40, p < .001). Dementia or cognitive impairment had a minor correlation with QoL (r = 0.30, p = .001). CONCLUSIONS: We found a high correlation between QoL and depression by using PDQ-39 and PDQL. In contrast, a minor correlation was observed between MMSE scores and health-related QoL. Treating depressive symptoms in PD patients may considerably improve the quality of life of patients with Parkinson's disease.