OBJECTIVE: To assess the effect of television viewing on subsequent change in body mass index (BMI=kg/m(2)) percentiles (BMI%) in adolescence. STUDY DESIGN: Data were drawn from the California Teen Longitudinal Survey of adolescents 12 to 17 years old with baseline assessment in 1993 and follow-up in 1996. Self-reported height and weight were used to calculate BMI and derive age-specific and sex-specific BMI%. Hours of television watched per day were obtained at baseline (BTV). The relations of BTV and BMI percentiles both at baseline and after 3 years were assessed with linear regression modeling. RESULTS: Of 2223 adolescents (52% male, 68% white), 5.85% (n=130) were overweight (BMI > or =95th percentile) at baseline and 5.40% (n=120) at follow-up. Mean BTV was 2.85 (SD, 1.98). In adjusted models, with each additional hour of BTV, the baseline BMI% increased by.9, and the follow-up BMI% increased by.47. Adolescents who watched more than 2 hours of television a day were twice as likely to be overweight at follow-up as adolescents who watched < or =2 hours. CONCLUSIONS: Television viewing leads to a subsequent increase in BMI percentiles and overweight. Efforts to decrease overweight should consider interventions to reduce television time.