OBJECTIVES--To develop and test a questionnaire for measuring quality of life in patients with Parkinson's disease. METHODS--An item pool was developed based on the experience of patients with Parkinson's disease and of neurologists; medical literature on the problems of patients with Parkinson's disease; and other quality of life questionnaires. To reduce the item pool, 13 patients identified items that were a problem to them and rated their importance. Items which were most often chosen and rated most important were included in the Parkinson's disease quality of life questionnaire (PDQL). The PDQL consists of 37 items. To evaluate the discriminant validity of the PDQL three groups of severity of disease were compared. To test for convergent validity, the scores of the PDQL were tested for correlation with standard indices of quality of life. RESULTS--The PDQL was filled out by 384 patients with Parkinson's disease. It consisted of four subscales: parkinsonian symptoms, systemic symptoms, emotional functioning, and social functioning. The internal-consistency reliability coefficients of the PDQL subscales were high (0.80-0.87). Patients with higher disease severity had significantly lower quality of life on all PDQL subscales (P < 0.05). Almost all PDQL subscales correlated highly (P < 0.001) with the corresponding scales of the standard quality of life indices. CONCLUSION--The PDQL is a relevant, reliable, and valid measure of the quality of life of patients with Parkinson's disease.