Objective: To determine the association of birth weight with obesity risk at first grade of high school in Chilean children after accounting for potential confounding factors. Design: National non-concurrent cohort of newborns. Sociodemographic information, height, weight and anthropometric measurements at first grade of high school were analysed. Birth weight was classified as macrosomia ($4000 g), by gestational age and by ponderal index. The relationship between birth weight and obesity at first grade of high school (BMI $ 95th percentile of the US Centers for Disease Control and Prevention's reference) was assessed using logistic regression models adjusted for sociodemographic information at delivery. Setting: First grade of public high school of low and middle socio-economic status in the whole country (about 77 % of Chilean children in this age group). Subjects: Newborns (n 119 070) and the same number of high-school students. Results: A positive relationship of high ponderal index (OR 5 1?86, 95 % CI 1?69, 2?03), birth weight $4000 g (OR 5 1?66, 95 % CI 1?54, 1?78) and large for gestational age (OR 5 1?69, 95 % CI 1?58, 1?81) with obesity at adolescence (P , 0?001) was found. Macrosomic children had a higher risk of being obese at first grade of high school after controlling for prenatal confounding variables (OR 5 1?63, 95 % CI 1?52, 1?76; P , 0?001). Conclusions: A direct relationship between high birth weight and obesity at first grade of high school was observed in this group of Chilean children. The results highlight the significance of birth weight as a simple tool to be used as an indicator of obesity risk for children by health-care providers.