Attention plays an important role in the gait disturbances of patients with Parkinson's disease (PD); however, the effects of "dual tasking" on fall risk and gait instability have not been well studied. The authors tested the hypothesis that gait variability increases when subjects with PD walk while performing a cognitively challenging task (CCT). Subjects with idiopathic PD walked under normal conditions and while performing a CCT. During normal walking, gait variability was significantly (.0001 < P < .05) correlated with fall risk, disease duration, disease severity, motor function, mentation, behavior and mood, and cognitive function. When walking while cognitively challenged, gait variability increased (from 47 +/- 29 msec to 223 +/- 281 msec, P < .002). During dual-task walking, only disease duration remained significantly associated with gait variability. These results highlight the profound effects of attention on gait and indicate that walking while cognitively challenged impairs the ability of patients with PD to maintain a stable walk.