CONTEXT: Whether 100% fruit juice consumption causes weight gain in children remains controversial. OBJECTIVE: To determine the association between 100% fruit juice consumption and change in BMI or BMI z score in children. DATA SOURCES: PubMed, Embase, CINAHL, and Cochrane databases. STUDY SELECTION: Longitudinal studies examining the association of 100% fruit juice and change in BMI measures were included. DATA EXTRACTION: Two independent reviewers extracted data using a predesigned data collection form. RESULTS: Of the 4657 articles screened, 8 prospective cohort studies (n = 34 470 individual children) met the inclusion criteria. Controlling for total energy intake, 1 daily 6- to 8-oz serving increment of 100% fruit juice was associated with a 0.003 (95% CI: 0.001 to 0.004) unit increase in BMI z score over 1 year in children of all ages (0% increase in BMI percentile). In children ages 1 to 6 years, 1 serving increment was associated with a 0.087 (95% confidence interval: 0.008 to 0.167) unit increase in BMI z score (4% increase in BMI percentile). 100% fruit juice consumption was not associated with BMI z score increase in children ages 7 to 18 years. LIMITATIONS: All observational studies; studies differed in exposure assessment and covariate adjustment. CONCLUSIONS: Consumption of 100% fruit juice is associated with a small amount of weight gain in children ages 1 to 6 years that is not clinically significant, and is not associated with weight gain in children ages 7 to 18 years. More studies are needed in children ages 1 to 6 years.