Objective: To investigate the patterns of physical activity (PA) and screen time among children and adolescents attended by the Brazilian National Health System. Methods: This cross-sectional study was conducted in the city of Bauru, São Paulo, Brazil. The sample was composed of 270 children and adolescents attending 5 basic health care units. PA and screen time were assessed through a questionnaire. Economic status, sex, and body composition variables were used as covariates. Results: Boys were more likely to be active when compared with girls (odds ratio = 2.56; 95% confidence interval, 1.43–4.58). Children and adolescents who accumulated less screen time were more likely to be in the most active group (odds ratio = 0.41; 95% confidence interval, 0.24–0.72). When comparing data year on year, we found that the level of PA has not changed significantly among children aged 7–10 years; however, at the age of 10, we noticed a significant reduction in PA scores, which kept decreasing until the age of 12. Regarding screen time, the situation was the opposite, with the amount of time spent in this behavior increasing over the years. Conclusion: Our results showed an association between low PA levels and high amount of screen time among children and adolescents, showing that is necessary to encourage an active lifestyle, decreasing time spent on television, computers, cell phones, and video games.