BACKGROUND: Attending childcare is related to greater childhood obesity risk, but there are few long-term follow-up studies. We aimed to examine the associations of childcare type, age at start, and intensity with body mass index body mass index (BMI) trajectories from ages 10 to 42 years. METHODS: The sample comprised 8234 individuals in the 1970 British Cohort Study, who had data on childcare attendance (no, yes), type (formal, informal), age at start (4-5, 3-3.99, 0-2.99 years old), and intensity (1, 2, 3, 4-5 days/week) reported at age 5 years and 32 563 BMI observations. Multilevel linear spline models were used to estimate the association of each exposure with the sample-average BMI trajectory, with covariate adjustment. A combined age at start and intensity exposure was also examined. RESULTS: Attending vs not attending and the type of childcare (none vs formal/informal) were not strongly related to BMI trajectories. Among participants who attended childcare 1 to 2 days a week, those who started when 3 to 3.99 years old had a 0.197 (-0.004, 0.399) kg/m2 higher BMI at age 10 years than those who started when 4 to 5 years old, and those who started when 0 to 2.99 years old had a 0.289 (0.049, 0.529) kg/m2 higher BMI. A similar dose-response pattern for intensity was observed when holding age at start constant. By age 42 years, individuals who started childcare at age 0 to 2.99 years and attended 3 to 5 days/week had a 1.356 kg/m2 (0.637, 2.075) higher BMI than individuals who started at age 4 to 5 years and attended 1 to 2 days/week. CONCLUSIONS: Children who start childcare earlier and/or attend more frequently may have greater long-term obesity risk.