An important criterion in scale validation is the demonstration of a stable factor structure. The Unified Parkinson's Disease Rating Scale (UPDRS) is widely used to assess Parkinson's disease (PD). The reliability and applicability of the motor subscale of the UPDRS (UPDRSm) when applied to patients diagnosed with progressive supranuclear palsy (PSP) is unknown. In a sample of 175 patients with PSP, factor analysis revealed five clinically distinct factors: two independent bradykinesia factors (axial/gait and extremities), one rigidity factor, and two independent tremor factors (rest and action). Two items (posture and rest head tremor) did not reach criteria for factor loadings. There was a high degree of internal consistency. These results suggest that UPDRSm is a reliable and applicable scale for assessing most aspects of PSP function as well as severity measures of five clinical disability domains.