['The objectives of this study were to (i) compare the strength of associations between sleep duration and body mass index (BMI) in middle childhood, and early and late adolescence; (ii) determine whether sleep duration in middle childhood predicts BMI in early or late adolescence; and (iii) examine the consistency of these associations by sex.', 'Subjects included 313 children/adolescents aged 8-19 participating in a longitudinal cohort study on sleep and health. Participants were assessed at three time points approximately 4\u2009years apart: ages 8-11, 12-15 and 16-19. BMI z-score (BMIz) was calculated using age and sex normative data from the Centers for Disease Control. Sleep duration was reported by the parent (ages 8-15) or the adolescent (ages 16-19).', '[corrected] Half of the participants were male and 79% were Caucasian. Sleep duration had a negative linear association with BMIz for boys but not girls, and the magnitude of this association decreased with age. Sleep duration at age 8-11 predicted BMIz in early and late adolescence for boys but not girls, and associations were largely attenuated after adjusting for BMIz at age 8-11. The strongest predictor of adolescent BMIz was BMIz at age 8-11 for both boys and girls.'] Does relation between sleep duration and BMI vary by age and sex in youth age 8-19?