Background: Assessment of implementation is essential for the evaluation of school-based preventive activities. Interventions are more easily implemented in schools if detailed instructional manuals, lesson plans, and materials are provided; however, implementation may also be affected by other factors than the intervention itself--for example, school-level characteristics, such as principal support and organizational capacity. We examined school-level characteristics of schools in groups of high, medium, and low implementation of a smoking prevention intervention. Methods: The X:IT study is a school-randomized trial testing a multicomponent intervention to prevent smoking among adolescents. Our data came from electronic questionnaires completed by school coordinators at 96.1% of participating intervention schools (N = 49) at first follow -up. Results: Schools that implemented the X:IT intervention to a medium or high degree had higher levels of administrative leadership (77.3% and 83.3% vs 42.9%), school climate/organizational health (95.5% and 91.7% vs 66.7%), mission-policy alignment (90.9% and 100.0% vs 71.4%), personnel expertise (81.8% and 75.0% vs 46.7%), school culture (77.3% and 91.7% vs 53.3%), positive classroom climate (91.4% and 96.2% vs 82.9%) compared with low implementation schools. Conclusion: Our findings highlight the importance of considering the school context in future health prevention initiatives.