The primary goals of the present study were 1) to explore the types and frequencies of adaptation reported by facilitators; 2) to document their reasons for making those adaptations; and 3) to propose an efficient approach to the study of prevention program fidelity based on interview data. We interviewed 42 program facilitators involved in a large-scale dissemination about their implementation of a community-based prevention program. Interview questions addressed facilitators' attitudes about program fidelity and the various types of changes, deletions, and additions they made. Although nearly all facilitators reported that fidelity to program curriculum was important, most also reported adapting the program. The most frequent reason facilitators gave for adaptation was deleting or changing material because they ran out of time. We employed a method used in business and health care settings (the Pareto principle, or the "law of the vital few") to identify the minority (30%) of types of adaptation that accounted for a majority (70%) of all adaptations reported. Similarly, we identified the minority of reasons (25%) for adaptation that accounted for a majority (75%) of reasons reported. This approach helps to focus the assessment of fidelity on those adaptations that occur with high frequency, since low-frequency events are unlikely to have a significant impact on large-scale outcomes. High-frequency adaptations can then be targeted to determine their effect on outcomes; to address in training; and to assess on an ongoing basis for continuous quality improvement.