INTRODUCTION: Parks provide opportunities for physical activity for children. This study examined sex differences in correlates of park-based physical activity because differences may indicate that a standard environmental intervention to increase activity among children may not equally benefit boys and girls. METHODS: The System for Observation Play and Recreation in Communities was used to measure physical activity among 2,712 children and adolescents in 20 neighborhood parks in Durham, North Carolina, in 2007. Sedentary activity, walking, vigorous park activity, and energy expenditure were the primary outcome variables. Hierarchical logit regression models of physical activity were estimated separately for boys and girls. RESULTS: Type of activity area and presence of other active children were positively associated with boys’ and girls’ physical activity, and presence of a parent was negatively associated. A significant interaction involving number of recreation facilities in combination with formal activities was positively associated with girls’ activity. A significant interaction involving formal park activity and young boys (aged 0-5 y) was negatively associated with park-based physical activity. CONCLUSION: Activity area and social correlates of park-based physical activity were similar for boys and girls; findings for formal park programming, age, and number of facilities were mixed. Results show that girls’ physical activity was more strongly affected by social effects (eg, presence of other active children) whereas boys’ physical activity was more strongly influenced by the availability of park facilities. These results can inform park planning and design. Additional studies are necessary to clarify sex differences in correlates of park-based physical activity.