The aim of the study presented here was to determine the relationship between age and certain socioeconomic factors and the effect that physical activity may have on the prevalence of overweight and obesity among Spanish children and adolescents. The sample derived from the enKid study, a cross-sectional study of a representative sample of the Spanish population aged 2-24 years. Body mass index was calculated from measured weight and height. A physical activity questionnaire was administered. Age, socioeconomic level of the families, the mother's level of education, and the geographical area of residence were related to the prevalence of overweight and obesity among Spanish males.