Lifestyle Education for Activity Program (LEAP) was a comprehensive, school-based intervention designed to promote physical activity in high school girls. The intervention focused on changes in instructional practices and the school environment to affect personal, social, and environmental factors related to physical activity. Multiple process evaluation tools and an organizational assessment tool were developed to monitor program implementation from a framework called the LEAP essential elements, which characterized complete and acceptable intervention delivery; secular trends were also monitored. Using process data, LEAP intervention schools were categorized into low- and high-implementing groups and compared with control schools on nine essential elements assessed at the organizational level. The Wilcoxon scores test revealed that low- and high-implementing intervention, and control schools differed significantly on two of nine administrator-reported organizational-level components: having a physical activity team and having a faculty-staff health promotion program. A mixed-model analysis of covariance indicated that, compared to control schools, a greater percentage of girls in high-implementing schools reported engaging in vigorous physical activity. Process evaluation can be used to understand the relationship between level of implementation and successful program outcome. (Contains 6 tables and 1 figure.)