We aimed to estimate the association between sleep duration trajectories and body composition in adolescents. We used data from participants of the 1993 Pelotas (Brazil) Birth Cohort Study who were later followed up at age 18 years (response rate of 81.3%). At the time, 3974 adolescents had complete data on body composition, which was assessed by air displacement plethysmography. Sleep duration was self-reported by participants at ages 11 and 18 years. Analyses were sex-stratified. The mean sleep duration at 11 years was 9.7 (SD 1.4) and 8.4 (SD 1.9) at 18 years. Sleep duration was dichotomized as inadequate (< 8 hours/day) or adequate (>= 8 hours/day). Mean body mass, fat mass, and fat-free mass indices at 18 years were 23.4 kg/m(2) (SD 4.5), 6.1 kg/m(2) (SD 3.9) and 17.3 kg/m(2) (SD 2.5), respectively. Girls who reported inadequate sleep duration at 11 years of age, but adequate sleep duration at 18, on average experienced an increase in body mass index (beta = 0.39 z-scores; 95% CI 0.13, 0.65), fat mass index (beta = 0.30 z-scores; 95% CI 0.07, 0.53), and fat free mass index (beta = 0.24 z-scores; 95% CI 0.08, 0.39) compared to those who had adequate sleep duration at both time points. The results suggest that changes in sleep duration across adolescence may impact body composition in later adolescence and that this may differ by sex.