In "a treatment effectiveness trial", a program is evaluated in a real-world setting, with an emphasis on achieving high implementation fidelity. Through fidelity assessment, the link between program implementation and outcomes is systematically evaluated and ultimately leads to a greater understanding of program success or failure. This paper reports the results of an implementation fidelity study of the "Reconnecting Youth" (RY) prevention program. The research questions were (i) was the program implemented with fidelity? and (ii) did better fidelity predict better outcomes? RY is an indicated drug abuse prevention program for high school students that seeks to "reconnect" high-risk youth to school before they drop out. The results reported here were part of a randomized controlled effectiveness trial of the RY prevention program conducted in two urban school districts in which 15 teachers taught 41 RY classes. Overall, implementation fidelity was high with an average 90% of core lessons being taught. Unexpectedly, increased quality of implementation predicted increased alcohol use and anger. Adherence (teaching more of the curriculum) predicted increased marijuana use, while exposure (student attendance) significantly increased bonding to high-risk peers, alcohol use and anger. (Contains 4 tables and 1 figure.)