OBJECTIVES: To examine the effects of duration, timing and type of television (TV) viewing at age 5 years on body mass index (BMI) in adult life. STUDY DESIGN AND METHODS: 1970 British Birth Cohort, followed up at 5 (N=13,135), 10 (N=14,875), and 30 years (N=11,261). OUTCOME MEASURES: Weekday and weekend TV viewing at 5 years, type of programs, and maternal attitudes toward TV at age 5 years. BMI z-score at 10 and 30 years. RESULTS: Mean daily hours of TV viewed at weekends predicted higher BMI z-score at 30 years (coefficient=0.03, 95% CI: 0.01, 0.05, P=.01) when adjusted for TV viewing and activity level at 10 years, sex, socioeconomic status, parental BMIs, and birth weight. Each additional hour of TV watched on weekends at 5 years increased risk of adult obesity (BMI >/=30 kg/m(2)) by 7% (OR=1.07, 95% CI 1.01, 1.13, P=.02). Weekday viewing, type of program and maternal attitudes to TV at 5 years were not independently associated with adult BMI z-score. CONCLUSIONS: Weekend TV viewing in early childhood continues to influence BMI in adulthood. Interventions to influence obesity by reducing sedentary behaviors must begin in early childhood. Interventions focusing on weekend TV viewing may be particularly effective