BACKGROUND: Obesity is an increasing problem so understanding the association between childhood and adolescent measures of body mass index (BMI) and being overweight at age 21 has implications for treatment or strategies to reduce its prevalence. OBJECTIVE: To examine the association between measures of BMI in childhood and adolescence and parents' BMI and being overweight at age 21. DESIGN: The study was based on a birth cohort born in Dunedin, New Zealand in 1972-1973. RESULTS: BMI tracked from childhood to early adulthood. The point on the BMI distribution where the probability of being overweight at age 21 was 0.5 was close to the 75th centile for boys throughout childhood and adolescence. It was rather higher for girls in childhood but similar in adolescence. Boys with a BMI above the 75th centile at age 7 were more than 4.0 times more likely to be overweight at age 21 than those with a BMI below the median. The relative risk for girls was 3.2. By age 15 this increased to 9.8 for males and 6.8 for females. Having overweight parents, particularly a mother, increased the likelihood of being overweight. Only 40% of those who were overweight at age 21 could be identified by age 7 and 25% were not identified until they were at least 15. CONCLUSION: Although a high BMI in childhood predicts being overweight at age 21, many of those who were overweight at age 21 had a BMI below the 75th centile or even the median in childhood and early adolescence. Population strategies, complemented by an individual approach for those above the 75th centile, are needed to reduce the average BMI of the population. International Journal of Obesity (2001) 25, 158-163 | 
