HIV prevention, by intervening within social networks, is potentially important but highly understudied. Approaches that systematically identify, train, and enlist known social influence leaders to advise members of their own networks in risk reduction constitute ways to reach hidden population segments, persons who are distrustful of authorities but trust their peers, and those who cannot be reached through traditional professionally delivered counseling. This article illustrates and provides evaluation data on a program that recruited 14 intact social networks of young men who have sex with men (YMSM) in St. Petersburg, Russia, and Sofia, Bulgaria. Sociometric measures were used to identify the social leader of each network, and baseline risk assessment measures were administered to all members of each social network. The sociometrically determined leaders then attended a six-session group program that provided training and guidance in how to carry out theory-based and tailored HIV prevention conversations with members of their own social networks. Four months after leaders completed the program, all network members were readministered risk assessment measures. Pre- to postintervention data revealed that the program produced: (1) increases in the level and comfort with which network members talked about AIDS prevention topics in their daily conversations; (2) increased network-level AIDS risk reduction knowledge and improved risk reduction norm perceptions, attitudes, behavioral intentions, and self-efficacy; and (3) increased condom use levels among network members. Although not a controlled, randomized trial, these program evaluation findings strongly support the feasibility of social network-level HIV prevention approaches.