Psychiatric, cognitive and sleep disorders are the most frequent and disabling non-motor complications of Parkinson's disease (PD). To improve the description of sleep and mental disorders in PD patients, we set out to develop a simple and reliable data collection tool (questionnaire) for the screening of large samples of PD patients. The first draft of the questionnaire was administered to a consecutive series of 120 PD patients from the outpatient department of our unit, who were instructed to fill it in with the help of their caregivers. Subsequent drafts of the questionnaire were evaluated together with the patients and their caregivers, until a final, satisfactory version was obtained. This final version was named the Questionnaire on Sleep and Mental Disorders in PD (QSMDPD). This questionnaire--we used the Italian version, named Questionario sui Disturbi del Sonno e Mentali nella Malattia di Parkinson, ODSMMP--consists of 119 questions with multiple-choice answers. The QSMDPD was mailed or handed to 400 PD patients followed at our unit's outpatient department. Three hundred and twenty (80%) were returned to us. A review of these completed questionnaires, conducted by a neurologist together with the patients, showed 90% of them (289) to be complete and to provide reliable data. This high compliance suggests that the QSMDPD is a promising tool for collecting data on sleep and mental disorders in large samples of PD patients. A short version will be administered as a follow-up tool.