Factors that influenced exercise behaviors and aerobic fitness were identified in 100 outpatients with rheumatoid arthritis or osteoarthritis. Data included perceived health status, benefits of and barriers to exercise, and impact of arthritis on health; demogaphic and biologic characteristics; and past exercise behavior. Excercise measures included range-of-motion and strengthening exerises, 7-day activity recall, and the exercise subscale of the Health-Promoting Lifestyle Profile. An aerobic fitness level was obtained on each subject by bicycle ergometer testing. The theoretical model predicted 20% of the variance in composite exercise scores but none of the variance in aerobic fitness levels. Perceived benefits of exercise was a significant predictor of exercise participation. Subjects with less formal education, longer duration of arthritis, and bigher impact of arthritis scores perceived fewer benefits of exercise, while subjects who reported exercising in their youth perceived more benefits of exercise.