There is now ample evidence that teachers tend to make substantial modifications to both the prescribed content and methods of the curricula they administer, and that such modifications are likely to attenuate curricula effects. We examine the fidelity with which teachers implement "Protecting You, Protecting Me," an underage alcohol use prevention curriculum. Findings suggest that while teachers attempted to implement most sections of a lesson, the lessons taught were consistently--and often extensively--adapted. We conclude that since teachers are likely to continue to modify lessons, curriculum developers and trainers should enhance their understanding of how prevention curricula are taught under real world conditions, help teachers to reinforce key curriculum concepts, and consider modifying those curricular sections that teachers are adapting with greatest frequency. (Contains 5 tables.)