Background: Health behavior theory generally does not include intrinsic motivation as a determinate of health practices. Purpose: The purpose of this study was to test the flow theory of exercise adherence. Flow theory posits that exercise can be intrinsically rewarding if the experiences of self/time transcendence and control/mastery are achieved during performance. It was hypothesized that higher levels of flow during exercise would be associated with exercise adherence rates. Methods: Expert panel review, test-retest, and factor analysis were used to establish the reliability and validity of a Flow for Exercise Adherence Scale. Free-living exercise among 445 young adults (age range 19-29) was tracked for 4 weeks to test the scale. Results: Flow scale scores differentiated between stages of change for exercise as theorized. Flow scale scores were significantly correlated with days of vigorous physical activity for subjects in the maintenance ("r" = 0.52) and action ("r" = 0.31) stages. Discussion: The results of this study are consistent with flow theory. Intrinsic motivation can be an important factor in free-living exercise adherence. Translation to Health Education Practice: Health educators may enhance program effectiveness by using flow theory to focus on intrinsic motivations for the target health behavior.