Objective: To assess associations between childhood body weight, weight gain during childhood to adolescence/young adulthood and incidence of adult metabolic syndrome (MetS). Design: A dynamic prospective cohort study (the SUN Project; Seguimiento Universidad de Navarra). Participants were asked to select which of nine body images most closely represented their body shape at ages 5 and 20 years, and it was used as a proxy of BMI. An incident case of MetS was diagnosed according to criteria of the International Diabetes Federation. Associations between childhood body weight, weight gain during childhood to adolescence/young adulthood and incidence of adult MetS were estimated by multiple-adjusted odds ratios and their 95 % confidence intervals. Setting: University of Navarra, Spain. Subjects: The study included 5317 university graduates, followed-up for a median of 6?1 years. Results: The incidence of MetS was 2?9 % (1?7 % in women and 5?1 % in men). Among men, body shape at age 5 years was inversely related to adult MetS (OR 5 0?83, 95 % CI 0?72, 0?97), whereas weight gain during childhood to adolescence/young adulthood was directly associated with adult MetS (OR 5 1?49, 95 % CI 1?01, 2?18); both childhood underweight (OR 5 5?20, 95 % CI 1?87, 14?50) and childhood obesity (OR 5 4?66, 95 % CI 1?40, 15?51) increased the likelihood of adult MetS. No association was apparent among women. Conclusions: These results support treating childhood underweight and weight gain during childhood to adolescence/young adulthood as part of comprehensive adult MetS prevention efforts in men.