BACKGROUND: A low glycemic index (LGI) diet has been proposed as a treatment for obesity in adults; few studies have evaluated LGI diets in obese children. AIM: The purpose of the study was to compare the effects of two diets, with similar energy intakes, but different glycemic indexes in a pediatric outpatient setting. SUBJECTS AND METHODS: A parallel- group, randomized controlled trial was conducted, and 22 obese outpatient children with a body mass index (BMI) Z-score >2 (11 females and 11 males, BMI 28.9±2.9 kg/m²) were included in the study. Patients were randomly allocated to a hypocaloric LGI (GI:60), or to a hypocaloric high glycemic index (HGI) diet (GI:90). The LGI and HGI diets were almost equivalent for macronutrient composition. Anthropometric and biochemical parameters were measured at baseline and after 6 months. RESULTS: In both groups there were significant decreases in BMI, BMI Z-score, blood pressure, and high-sensitivity C-reactive protein. Only LGI diets produced a significant decrease in waist circumference and homeostasis model assessment. Analysis of variance demonstrated that the BMI Z-score decrease from baseline values was significantly greater after the LGI diet than after the HGI diet [-0.20 (95% confidence interval (CI) -0.29 to -0.10) vs -0.34 (95%CI -0.43 to -0.24)], mean difference between groups -0.14 (95%CI -0.27 to -0.01), p<0.05). Changes in triglyceride concentrations were significantly lower in LGI as compared to HGI diet (p<0.05). CONCLUSIONS: This study demonstrates that a hypocaloric LGI diet has beneficial metabolic effects in comparison to a hypocaloric HGI diet in obese children.