BACKGROUND: Parks have been proposed as a feature of the built environment that may promote increased physical activity. Little, if any, research has investigated the role of the park social environment in promoting physical activity within parks, however. PURPOSE: To examine whether social capital is a collective feature of the park environment and whether it is associated with park use and park-based physical activity. METHODS: Adult park users (n=222) were surveyed in 27 neighborhood parks in New Orleans LA in July-August 2008. Direct observation methods were used to count the numbers and activity levels of all park users in these parks on weekdays during the hours of 4:00-7:00pm. Multilevel linear regression models were used to calculate the intraclass correlation (ICC), which measures the variation in perceived social capital attributable to differences among parks and to test whether park use and physical activity outcomes differed between parks with high versus low levels of social capital. Analyses were conducted in 2009-2010. RESULTS: In study parks, 27% of perceived social capital was attributable to differences among parks (ICC=0.27). Parks with higher levels of social capital had higher daily numbers of observed park users (42.5 vs 12.1, p=0.0044) and had more energy expended within the park (3200.3 vs 721.2 MET-minutes across all park users, p=0.0087). CONCLUSIONS: Interventions to improve park social environments should be conducted to determine if they promote increased physical activity among park users.