Background and Purpose. The Timed “Up & Go” Test (TUG) is used to measure the ability of patients to perform sequential locomotor tasks that incorporate walking and turning. This study investigated the retest reliability, interrater reliability, and sensitivity of TUG scores in detecting changes in mobility in subjects with idiopathic Parkinson disease (PD). Subjects. The performance of 12 people with PD was compared with that of 12 age-matched comparison subjects without PD. Methods. The subjects with PD completed 5 trials of the TUG after withdrawal of levodopa for 12 hours (“off” phase of the medication cycle) as well as an additional 5 trials 1 hour after levodopa was administered (“on” phase of the medication cycle). They were scored on the Modified Webster Scale at both sessions. The comparison subjects also performed 5 TUG trials. All trials were videotaped and timed by 2 experienced raters. The videotape was later rated by 3 experienced clinicians and 3 inexperienced clinicians. Results. For the subjects with PD, within-session performance was highly consistent, with correlations (r) ranging from .80 to .98 for the “off” phase and from .73 to .99 for the “on” phase. The performance of the comparison subjects across the 5 trials was also highly consistent (r=.90–.97). Comparisons showed differences between trials 1 and 2 on the TUG for both groups. Removal of data for trial 1 (the practice trial) further enhanced retest reliability. There was close agreement in TUG scores among raters despite different levels of experience (intraclass correlation coefficient [3,1]=.87–.99). Mean TUG scores were different between the “on” and “off” phases of the levodopa cycle and between subjects with PD and comparison subjects during the “on” phase. Conclusion and Discussion. Retest reliability and interrater reliability of the TUG measurements were high, and the measurements reflected changes in performance according to levodopa use. The TUG can also be used to detect differences in performance between people with PD and elderly people without PD.