OBJECTIVE: To compare the performance of people with Parkinson's disease (PD) and unimpaired participants on a timed dexterity task and to examine the effects of adding a secondary task. DESIGN: A repeated measures analysis of performance for the 2 groups under unitask and dual-task conditions. SETTING: All tests were conducted in a human movement laboratory. PARTICIPANTS: People with idiopathic PD (n=22) and age-matched and sex-matched comparisons (n=22) volunteered for the study. INTERVENTIONS: Not applicable, although a verbal-cognitive secondary task was used. MAIN OUTCOME MEASURES: The number of pegs placed in the Purdue Pegboard in 30 seconds, the number of correct verbal responses for the secondary task, scores on the Manual Ability Measure-16 test of hand function and, for the group with PD, ratings on the Unified Parkinson's Disease Rating Scale. RESULTS: For the pegboard task, people with PD had reduced dexterity (t=-5.289; P<.001) compared with the unimpaired group. When the secondary task was added, both groups placed fewer pegs (F(1,42)=.652; P=.42). There were no differences between groups in scores for the subtraction task performed alone, but when this activity was carried out with the Purdue Pegboard Test, the number of correct responses declined only in the PD group (F(1,42)=4.90; P=.032). CONCLUSIONS: Manual dexterity was compromised in this group of people with mild-moderate PD when compared with an unimpaired group. When the concurrent verbal-cognitive task was added, dual-task interference occurred in both groups but to a greater extent in people with PD.