Background A high prevalence of obesity is the main public health problem in Chilean school children. Aim: To compare the nutritional status, consumption of selected foods and extracurricular physical activity (PA) habits in school children of different socioeconomic levels as a baseline for developing effective educational interventions. Material and methods: Cross-sectional study that determined the body mass index, food consumption and physical activity with previously validated instruments in 202 and 358 girls from 3(rd) to 8(th) grade in schools of medium-high and low socioeconomic level (SEL) from Santiago, Chile, respectively. Results: Compared to their counterparts of low socioeconomic level (SEL), the prevalence of obesity was significantly lower in 8-9 year-old girls of medium high SEL (19% and 9%, respectively, p =0.012) and 12-13 year=old (12% and 2.5% respectively, p =0.008). Also median daily intake of dairy products was higher in girls of medium high SEL (250 and 470 ml/day, respectively). The intake of fruits and vegetables was similar (200 g/d); and the intake of bread was lower (230 and 70 g/day, respectively, p <0.01). Consumption of energy-dense foods was lower in 10-13 year-old girls of medium high SEL (80 and 50 g/day, respectively, p <0.01). 45% of 8-9 year-old girls and 35% of 12-13 year-old girls of both SEL engaged in PA four or more times per week (NS). Conclusions: Although the prevalence of obesity in girls of medium-high SEL was not as high as in those from low SEL, it is still high. There is a need for educational interventions to improve their food and PA habits and to promote an environment that enhances healthy behaviors.