OBJECTIVE The purpose of the present study was to analyze the relationship between physical activity (PA) and obesity in Portuguese children and adolescents. METHODS The sample consisted of 1341 children and adolescents (8-15 years of age), 634 males (age, 10.6+/-2.3 y; body mass, 40.6+/-12.7 kg; height, 1.431+/-1.41 m) and 707 females (age, 10.9+/-2.4 y; body mass, 50+/-12.4 kg; height, 1.428+/-1.32 m). The sample was divided into quartiles of percentage of body fat (%BF) and physical activity index (PAI), within age and gender. Children in the upper %BF quartile were defined as obese (arbitrary definition). Logistic regression for each PAI quartile was used to determine the odds ratio of obese children and adolescents (> or = P75 of %BF) in comparison to their non-obese counterparts (< P75 of %BF). RESULTS Males in the lowest PAI quartile have an odds ratio of 2.1 of having obesity in relation to males in the highest PAI quartile. For females any significant result was found between PAI quartiles. CONCLUSIONS The results obtained in this study add some data to the controversy of PA and obesity relationship youth. Logistical regression shows that males who have a lower PAI have an odds ratio of 2.1 of obesity, in comparison with males that have higher PAI. Those findings were not found in girls.