BACKGROUND: The development of body weight is determined by different factors, namely genetic, behavioral, parental and physiological. OBJECTIVE: To investigate whether genetic, behavioral, parental and physiological factors are involved and the extent of involvement in the development of body weight at ages 12 and 13 y in a Dutch children cohort. METHODS: In a Dutch cohort of 94 children at ages 12 and 13 y, we determined anthropometric measurements, body composition, leptin concentrations, TFEQ scores, physical activity, as well as 3 polymorphisms, and in the parents we determined anthropometric measurements and TFEQ scores. RESULTS: 11% of the children in the cohort were classified as overweight. The genotype frequency distributions of the PPARy2, GRL and CNTF genes at ages 12 and 13 y were not significantly different for the overweight children compared to the lean children. Overweight children showed higher dietary restraint and disinhibition scores. Overweight children's parents had a higher BMI, dietary restraint and disinhibition scores, compared to lean children's parents. A peak in leptin concentrations between 7 and 13 y was shown at 12 y. In lean boys, the decrease in leptin concentrations between 12 and 13 y was related to an increase in fat free mass. At the age of 12 y predominantly the physiological factors were predictors for body weight, and at the age of 13 y both the physiological and behavioral factors were predictors for body weight. CONCLUSION: We conclude from this longitudinal study, that leptin appeared to play an important role in the development of body weight during puberty, in addition to behavioral and parental factors.