BACKGROUND Physical inactivity is an important health risk factor that could be addressed at the community level. PURPOSE The goal of the study was to determine whether using a community-based participatory approach with park directors and park advisory boards (PABs) could increase physical activity in local parks. Whether involving PABs would be more effective than working with park directors alone was also tested. DESIGN An RCT intervention from October 2007 to April 2012 was used, with partial blinding of observers to the condition. All data were analyzed in 2012. SETTING/PARTICIPANTS Of 183 eligible parks in the City of Los Angeles, 50 neighborhood park/recreation centers serving diverse populations participated. Parks were randomized to three study arms: (1) park-director intervention (PD-only); (2) PAB intervention (PAB/PD); and (3) a control arm. Physical activity in each park was systematically observed, and park users and residents living within 1 mile of the park were interviewed. INTERVENTION(S) The intervention included assessing park use, obtaining feedback from park users and community residents, training on outreach and marketing, and giving each intervention park $4000 to increase park-based physical activity. The PAB/PD arm required participation and concurrence on all purchases by the PAB. MAIN OUTCOME MEASURE(S) Change in the number of park users and change in the level of park-based physical activity, expressed as MET-hours. RESULTS Relative to control parks where physical activity declined, in both the PD-only and PAB/PD parks, physical activity increased, generating an estimated average of 600 more visits/week/park, and 1830 more MET-hours of physical activity/week/park. Both residents and park users in the intervention arms in the intervention arms reported increased frequency of exercise. No differences were noted between the PD-only and PAB/PD study arms. CONCLUSIONS Providing park directors and PABs with training on outreach and marketing, feedback on park users, and modest funds increased the amount of physical activity observed in parks.