Objective To evaluate an incentive-based physical activity intervention to increase physical activity and fitness among children in a 9-month cluster randomized controlled trial. Study design Children aged 6-12 years were randomized to control (n = 138 from 106 families) or intervention arm (n = 147 from 106 families). The intervention included incentives for meeting step targets as measured by pedometers and structured weekend outdoor activities. Outcomes included trends in activity for the intervention group and between-group differences in pedometer steps, 6-minute walk test, body mass index, and parent-reported Pediatric Quality of Life Inventory. Results At follow-up, children in the intervention group recorded significantly more pedometer steps than controls over the entire week (8660 vs 7767; P = .010), on weekdays (8646 vs 7826; P = .041), and on weekends (8779 vs 7684; P = .018). Three different trajectory classes were identified. The first group increased activity but was not sustained, the second group met the target step levels, and the third group significantly surpassed the step goals. The intervention group showed trends toward longer 6-minute walk test times and higher Pediatric Quality of Life Inventory scores, but the differences were not statistically significant. Conclusion Incentives for increased step activity were effective in producing greater steps and showed a (nonsignificant) trend toward improvements in other health outcomes. Thus, future incentive trials should be incorporate greater step targets and longer follow-up periods to provide evidence of the long-term effect of these incentives on children's health.