OBJECTIVE: To test the hypothesis that individuals who are active but who decrease physical activity (PA) over time have a higher risk of becoming obese in young adulthood, when compared to individuals who are consistently active throughout childhood and adolescence. METHODS: Iowa Bone Development Study cohort members (242 males and 251 females) participated in accelerometry assessments, dual-energy X-ray absorptiometry scans, and dietary questionnaire surveys at ages 5, 8, 11, 13, 15, 17, and 19 years. Group-based trajectory analyses identified distinct trajectory patterns of moderate- to vigorous-intensity PA (MVPA), percentage of body fat, and energy intake. A multivariable logistic regression model was fit to estimate the odds of "becoming obese" based on the MVPA trajectories, adjusted for mother's education, somatic maturation, and energy intake. RESULTS: Among males, 74.7% had a "normal" body fat pattern, 14.6% had a "becoming obese" pattern, and 10.7% had a "consistently obese" pattern, while among females, the percentages were 58.6%, 28.6%, and 12.8%, respectively. Participants who were active (>/=45 min MVPA) as children but decreased MVPA with age were more likely to become obese, compared to consistently active participants (adjusted OR = 2.77; 95% CI = 1.16, 6.58). CONCLUSIONS: An active lifestyle throughout childhood and adolescence could prevent obesity development in young adulthood.