The aim of this study was to examine the quantitative relationship between activity monitor measures and clinical scores of patients with Parkinson disease (PD). Motor activity was recorded continuously for 5 to 6 days at home with a wrist-worn activity monitor in 69 PD patients and 59 healthy controls. Clinical scores of the patients, age, and sex were submitted to multiple regression analysis to examine the quantitative relationship with measures reflecting the activity level and the proportion of activity and immobility over time. The patients' age, sex, and scores representing hypokinesia and rigidity and resting tremor explained approximately 50% of the variance of the motor activity measures. All motor activity measures declined with age; the rate of decline was similar for the patients and controls. Sex emerged as a predictor of the motor activity measures in the patients only. Male patients with PD showed significantly lower values for all motor activity measures than female patients and controls. Our results show that activity monitoring can be used as an objective quantitative assessment in PD.