This study tested the ability of the theory of planned behavior to predict actual participation in physical activity and explored the development of activity habits in a 12-week longitudinal study. People enrolling in a gymnasium (N = 94) completed standard theory of planned behavior measures at baseline and follow-up; behavior was monitored objectively in the intervening period. The data were analyzed by using both standard and repeatable events survival analysis. Results showed that (a) perceived behavioral control was significantly predictive of intentions and actual behavior, (b) stable exercise habits developed in the first 5 weeks of the study, and (c) successful prior performance enhanced perceptions of behavioral control. The implications for developing theory-based interventions that promote the maintenance of health behavior are discussed.((c) 2005 APA, all rights reserved).