The aim of this study was to evaluate the impact of an intervention program on the patterns of physical activity in 8- to 10-year-old Mexican children from lower socioeconomic status. This study performed a randomized controlled field trial in 498 children aged 8-10 years from 10 public schools of low socioeconomic status in Mexico City. Schools were randomly assigned to intervention (n = 5) or control (n = 5) groups and followed up during 12 months. Physical and sedentary activities were assessed at the beginning of the program and after 6 and 12 months. At the end of follow-up, there was a significant increase in the performance of moderate physical activity (MPA) among children in intervention group who had not performed MPA at baseline any day of the week (40%, P = 0.04) but not in the control group (8%, P = not significant). The intervention group also showed a significant reduction in the proportion of children who spent more than 3 hours a day playing video games (from 23 to 13%, P = 0.01), while control group did not show significant changes. Given these findings, we conclude that intervention was able to modify positively physical activity and reduce time spent on such sedentary activities as video games among those at highest risk studied children.