HPTN 037 was a Phase III, multi site, two-arm, randomized controlled trial conducted in Philadelphia, Pennsylvania, and Chiang Mai, Thailand, to determine the efficacy of a network-oriented peer educator intervention for prevention of HIV infection among injection drug users and members of their HIV risk network through reduction of HIV risk behaviors. Approximately 2,610 HIV uninfected injection drug users (index participants) and individuals identified by each index participant as members of their sex and drug-using networks (network members). Index participants and their network members were randomized to one of two groups in a ratio of 1:1. Index participants and network members in both arms received basic HIV counseling and testing enhanced with interactive risk reduction counseling. In addition, index participants randomized to the experimental arm received peer educator training.