OBJECTIVE: To evaluate the impact of a school-based obesity prevention program that seeks to change food intake among students at schools in Rosario, Argentina. METHODS: This was a prospective study involving 405 children 9-11 years of age at six schools in the poor areas of Rosario, Argentina, in May-October 2008. After matching for socioeconomic status, schools were selected by simple randomization; participants were assessed at baseline (T1) and again 6 months later, after completion of the intervention (T2). The program focused on increasing the children's knowledge of healthy nutrition and exercise through four workshops; educating the parents/caregivers; and offering healthy options at the school snack bar. The main outcome measures were the children's intake of healthy and unhealthy foods (assessed with a weekly food frequency questionnaire) and their body mass index (BMI). RESULTS: Of the 387 children assessed at T1, 369 were reassessed at T2 (205 intervention; 164 control). Girls at the schools where the intervention occurred increased their intake of three of the five healthy food items promoted by the program (fruits, vegetables, low-sugar cereals). Statistical significance was reached for skim milk (P = 0.03) and for pure orange juice (P = 0.05). Boys of both the intervention and control groups failed to improve their intake of healthy foods, but those of the intervention arm significantly reduced their intake of hamburgers and hot dogs (P = 0.001). CONCLUSIONS: Girls were more amenable to improving their dietary intake. Overall, the program was more likely to increase consumption of healthy food than to decrease intake of unhealthy foods. Gender differences should be taken into account when designing preventive interventions.