Objective - To evaluate the interference effects on walking of functional activities in the home in people with Parkinson’s disease (PD) and the contribution of clinical symptoms to disturbance of gait. Design - A repeated-measures trial, using a dual-task paradigm to evaluate the attentional demands of functional activities. Setting - Participants’ homes and a clinic. Participants - Twenty subjects with idiopathic PD and 10 age-, sex-, and education-matched controls. Interventions - Subjects performed a simple walking task, a dual-motor task, a dual-cognitive task, and a multiple task, all of which were real-world activities. Main outcome measures - Walking speed, mean step length, and step frequency were compared across different tasks for each subject. A battery of clinical outcome measures hypothesized to compete for attention were also conducted: cognition (Hayling and Brixton tests), anxiety and depression (Hospital Anxiety and Depression test), fatigue (Multidimensional Fatigue Inventory), balance (Berg Balance Scale), and disease severity (Hoehn and Yahr score). Results - PD subjects walked at a significantly slower speed (26.5%, P<.001) and reduced step length (23%, P<.001) than did the controls. Performance of a concurrent cognitive and multitask resulted in significantly slower gait speed (P=.022; P<.015) and reduced mean step length (P=.022; P=.001) in PD subjects. Cognitive function, depression, physical fatigue, and balance were significantly related to walking speed for the functional tasks. Multiple regression analysis showed that the Brixton test, physical fatigue, and depression accounted for up to 39% of the variation in walking speed during functional tasks in PD and control subjects and balance accounted for 54% of variance for walking in PD subjects. Conclusions - Competition for attention through additional activities, decreased executive function, depression, fatigue, and impaired balance will increase difficulty in walking for PD subjects. Evaluation of performance during complex functional activities in an appropriate environment should be a focus of therapeutic assessment. Furthermore, functional performance may be influenced by several other symptoms that should also be considered.