Context: Conventional dietary approaches for the treatment of obesity have generally yielded disappointing results. Objective: To examine the effects of a low-glycemic index (GI) diet compared with a standard reduced-fat diet in the management of pediatric obesity. Design: Retrospective cohort study of children attending an outpatient pediatric obesity program from September 1997 to December 1998. Setting: Academic medical center. Participants: One hundred seven obese but otherwise healthy children. Main Outcome Measures: Changes in body mass index (BMI [calculated as weight in kilograms divided by the square of height in meters]) and body weight from first to last clinic visit. Results: A total of 64 patients received the low glycemic index diet and 43 received the reduced-fat diet for 4.3 vs 4.2 months' mean duration of follow-up, with 3.3 vs 3.3 mean number of visits, respectively. Body mass index (-1.53 kg/m2 [95% confidence interval, -1.94 to -1.12] vs -0.06 kg/m2 [-0.56 to +0.44], P<.001) and body weight (-2.03 kg [95% confidence interval -3.19 to -0.88] vs +1.31 kg [-0.11 to +2.72], P<.001) decreased more in the low-GI group compared with the reduced-fat group. In multivariate models, these differences remained significant (P<.01) after adjustment for age, sex, ethnicity, BMI or baseline weight, participation in behavioral modification sessions, and treatment duration. Significantly more patients in the low-GI group experienced a decrease in BMI of at least 3 kg/m2 (11 kg/m2 [17.2%] vs 1 kg/m2 [2.3%], P=.03). Conclusions: A low-GI diet seems to be a promising alternative to standard dietary treatment for obesity in children. Long-term randomized controlled trials of a low-GI diet in the prevention and treatment of obesity are needed.