OBJECTIVE: To use epidemiological data and a standardized economic model to compare projected costs for obesity prevention in late adolescence accrued using a cross-sectional weight classification for selecting adolescents at age 15 years compared with a longitudinal classification. METHODS: All children born in a Swedish county (population 440 000) in 1991 who participated in all regular measurements of height and weight at ages 5, 10 and 15 years (n ¼ 4312) were included in the study. The selection strategies were compared by calculating the projected financial load resulting from supply of obesity prevention services from providers at all levels in the health care system. The difference in marginal cost per 1000 children was used as the primary end point for the analyses. RESULTS: Using the cross-sectional selection strategy, 3.8% of adolescents at age 15 years were selected for evaluation by a pediatric specialist, and 96.2% were chosen for population-based interventions. In the trajectory-based strategy, 2.4% of the adolescents were selected for intensive pediatric care, 1.4% for individual clinical interventions in primary health care, 14.0% for individual primary obesity prevention using the Internet and 82.1% for population-based interventions. Costs for the crosssectional selection strategy were projected to USD463 581 per 1000 adolescents and for the trajectory-based strategy were USD 302 016 per 1000 adolescents. CONCLUSIONS: Using projections from epidemiological data, we found that by basing the selection of adolescents for obesity prevention on weight trajectories, the load on highly specialized pediatric care can be reduced by one-third and total health service costs for obesity management among adolescents reduced by one-third. Before use in policies and prevention program planning, our findings warrant confirmation in prospective cost-benefit studies.