Background: Research has shown that participating in activities outside of school hours is associated with lower dropout rates, enhanced school performance, improved social skills, and reduced problem behaviors. However, most prior studies have been limited to small populations of older children (greater than 12 years). This analysis focuses on children aged 6 to 11 to assess the potential association between participation in activities outside of school hours and behavior in middle childhood in a nationally representative survey. Methods: Estimates were based on 25,797 children from the 2003-2004 National Survey of Children's Health. Outside of school activity was defined as participating in sports teams/lessons, clubs/organizations, or both at least once in the past year. Analysis of variance was used to evaluate the differences in behavior problems and social skills adjusting for sociodemographic factors, among children classified by participation in outside of school activities. Results: Seventy-five percent of children participated in outside of school activities: 23% in sports, 16% in clubs, and 36% in both clubs and sports. Activity participation differed by gender, race/ethnicity, type of school, poverty status, family structure, household education, and school and community safety. Children participating in both sports and clubs had higher social skills index scores, but no significant difference in problem behavior scores compared with children who did not participate in any outside of school activity. Conclusion: Children participating in both sports and clubs had greater social competence during middle childhood compared with children who did not participate in any outside of school activities. (Contains 2 tables and 1 figure.)