OBJECTIVES: We evaluated the use of social networks to reach persons with undiagnosed HIV infection in ethnic minority communities and link them to medical care and HIV prevention services. METHODS: Nine community-based organizations in 7 cities received funding from the Centers for Disease Control and Prevention to enlist HIV-positive persons to refer others from their social, sexual, or drug-using networks for HIV testing; to provide HIV counseling, testing, and referral services; and to link HIV-positive and high-risk HIV-negative persons to appropriate medical care and prevention services. RESULTS: From October 1, 2003, to December 31, 2005, 422 recruiters referred 3172 of their peers for HIV services, of whom 177 were determined to be HIV positive; 63% of those who were HIV-positive were successfully linked to medical care and prevention services. The HIV prevalence of 5.6% among those recruited in this project was significantly higher than the approximately 1% identified in other counseling, testing, and referral sites funded by the Centers for Disease Control and Prevention. CONCLUSIONS: This peer-driven approach is highly effective and can help programs identify persons with undiagnosed HIV infection in high-risk networks.