Our objective was to assess the test–retest reliability of the Unified Parkinson's Disease Rating Scale (UPDRS). The UPDRS is the most widely used instrument for measuring severity of parkinsonian symptoms in clinical research and in practice. The validity and inter-rater reliability of this scale have been previously studied. We examined the test–retest (intrarater) reliability of the UPDRS and derived subscales. Four hundred patients with early-stage Parkinson's disease (PD) who were participating in a multicenter clinical trial were evaluated using the UPDRS on two separate occasions (screening and baseline visits) prior to receiving treatment. The same neurologist at each center rated the subjects at both examinations that were, on average, 14.6 ± 7.6 days apart (range 3–36 days). Test–retest reliability was estimated using the intraclass correlation coefficient (ICC) for the total UPDRS score, the mental, ADL, and motor subscale scores, and other derived subscale scores. Weighted kappa statistics were calculated for individual UPDRS items. The ICCs for the UPDRS scores were as follows: total score, 0.92; mental, 0.74; ADL, 0.85; motor, 0.90. ICCs for derived symptom-based scales ranged from 0.69–0.88. Reliability of specific items was generally lower than for summary scales. Reliability was slightly better in patients for whom the testing interval was within 14 days. Based on conventional standards, the UPDRS scores were found to have excellent test–retest reliability in this sample of patients with early PD rated by academic movement disorder specialists. The findings are in agreement with previous reports on interrater reliability. © 2002 Movement Disorder Society.