Background: Short sleep duration is associated with obesity during childhood and adulthood. Objective: The objective of our study was to investigate the relationship between sleep duration and body mass index (BMI) from Tanner stages 1 to 5 in a Dutch children cohort. Design: In 98 children, anthropometric measurements and leptin concentrations were measured from age 7 to 16 years; body composition, physical activity (Baecke questionnaire), hours television viewing and self-reported sleep duration were measured yearly from age 12 to 16 years. Moreover, the polymorphisms of the FTO gene (rs9939609) and parental BMI's were determined. Results: At Tanner stages 1-5 sex differences were observed in height, body weight, waist circumference, fat mass per squared meter height and leptin concentrations per kg fat mass. Inverse relationships were observed between the change in BMI (kg m -2 ) and the change in hours of sleep per night (h) from Tanner stages 1 to 4 (r ¼ À0.68, Po0.001), from Tanner stages 2 to 5 (r ¼ À0.35, Po0.05) and from Tanner stages 1 to 5 (r ¼ À0.33, Po0.05). Univariate analysis of variance showed that with progressive Tanner stages, BMI increases and sleep duration decreases in an interrelated way independent of possible confounders (R 2 ¼ 0.38, Po0.02). Conclusion: Changes in BMI during puberty were inversely related to changes in sleep duration, independent of possible confounders.