The Unified Parkinson's Disease Rating Scale (UPDRS) is widely used to assess Parkinson's disease (PD) disability but its metric properties have not been extensively studied. We investigated the factor structure and internal consistency of the Motor Examination section of the UPDRS in a sample of 294 consecutive patients with idiopathic PD who were assessed while in the "on" state. There was a high degree of internal consistency. Factor analysis revealed six clinically distinct factors, three bradykinesia measures (axial/gait, right and left), one rigidity measure, and two tremor measures (rest and postural). These factors accounted for 78% of the variance. Total Motor Examination scores and selected factors correlated well with other examiner-determined global ratings of PD disability (Hoehn and Yahr stage and Schwab-England Activities of Daily Living score). These results suggest that the Motor Examination section of the UPDRS provides a useful measure of PD function as well as severity measures of six clinical disability domains.