Evidence is building regarding the association between inadequate amounts of sleep and the risk of obesity, especially in younger children. Less is known about the relationship between change in sleep and change in weight during adolescence. The objective of this study was to examine the longitudinal relationship between change in sleep duration and change in body mass index (BMI) and percent body fat (PBF) in a cohort of adolescents. The cohort included 723 adolescents (mean age = 14.7 years at baseline) from Minnesota. Total sleep duration was assessed via self-report. BMI and PBF were objectively assessed. Covariates used in the multivariate analyses included energy intake as assessed through 24-hour recalls, activity levels as assessed by accelerometers, screen time/sedentary behavior, depression, and sociodemographic characteristics. For both males and females, average BMI and PBF increased slightly over the 2 years and average sleep duration decreased by about 30 minutes. The authors saw no statistically significant longitudinal relationships between change in total sleep and change in BMI or PBF over time in either girls or boys. The only longitudinal relationship that approached statistical significance was a positive association between sleep and PBF in females ("p" = 0.068). This research contributes to the literature as the only study to date to examine how change in sleep duration during adolescence may be related to a concomitant change in BMI and body fat. The findings of this study do not support the hypothesis that a decline in sleep duration during adolescence increases obesity risk. (Contains 2 tables.)