Background. The benefits of exercise for reducing disability in people with Parkinson disease (PD) are becoming more evident. Optimal benefit, however, requires regular and sustained participation. Factors associated with engaging in regular exercise have received little scientific scrutiny in people with PD. Objective. The purpose of this study was to explore factors associated with exercise behavior in patients with PD using the International Classification of Functioning, Disability and Health (ICF) as a guiding framework. Design. This was a cross-sectional study. Methods. The participants in this study were 260 patients with PD from 4 institutions. Participants were designated as "exercisers" or "nonexercisers" based on responses to the Stages of Readiness to Exercise Questionnaire. Exercise status was validated using the Physical Activity Scale for the Elderly and an activity monitor. Factors potentially associated with exercise behavior included measures of body structure and function, activity, participation, environmental factors, and personal factors. Their relative contributions were analyzed using logistic regression and quantified with odds ratios. Results. One hundred sixty-four participants (63%) were designated as exercisers. Participants with high self-efficacy were more than twice as likely to engage in regular exercise than those with low self-efficacy (adjusted odds ratio 2.34,95% confidence interval 1.30-4.23). College educated and older participants also were more likely to exercise. Disabling influences of impairments, activity limitations, and participa- tion restrictions were not associated with exercise behavior. Limitations. The cross-sectional nature of the study limited the ability to make causal inferences. Conclusions. Self-efficacy, rather than disability, appears to be strongly associ- ated with whether ambulatory, community-dwelling people with PD exercise regu- larly. The results of this study suggest that physical therapists should include strate- gies to increase exercise self-efficacy when designing patient intervention programs for patients with PD.