To test for an association between the LRRK2-G2019S mutation and gait, we studied 52 first-degree relatives of patients with Parkinson's disease (PD) who carry this mutation. An accelerometer quantified gait during usual-walking, fast-walking, and dual-tasking. Noncarriers (n = 27) and carriers (n = 25) were similar with respect to age, gender, height, and gait speed during all conditions. During dual-tasking and fast-walking, gait variability and the amplitude of the dominant peak of the accelerometer signal were significantly altered among the carriers. These findings support the possibility of previously unidentified, presymptomatic motor changes among relatives who have an increased risk of developing PD. Ann Neurol 2010