Background: This study consisted of a formative evaluation of an after-school health education program designed for adolescent females, entitled Girls on Track. Evidence-based after-school programs have potential to supplement the traditional school day, encourage social and emotional skill development, improve the quality of student health, and contribute to a healthier school environment. Implementing comprehensive programs outside of the classroom, however, is challenging and gaps exist in the literature regarding implementation fidelity. Methods: The national program was in a cohort of adolescent females from 11 Girls on Track locations in fall 2008 and 10 locations in spring 2009. Mixed-method analyses evaluated fidelity of lesson implementation, described curricular and programmatic characteristics associated with implementation fidelity, and assessed coach perceptions toward program quality and feasibility. Results: Specific lesson characteristics associated with higher rates of implementation fidelity and favorable coach perceptions toward program feasibility included improved lesson clarity, more time to process health topics, teaching and learning objectives that were well aligned with lesson activities, incorporation of alternative activities for less-motivated participants, and provision of resources on key health topics for coaches. Conclusion: Girls on Track is currently being implemented across the United States and Canada, reaching over 5000 adolescent females annually. Identifying and incorporating specific curricular and programmatic characteristics associated with high levels of implementation fidelity can enhance the quality and benefits of after-school programs. (Contains 1 table.)