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.