Men who have sex with men (MSM) are disproportionately affected by HIV, and HIV-seropositive (HIV-positive) MSM are an especially important group for prevention efforts. This article describes findings from the Seropositive Urban Men's Study (SUMS, N = 456) and the Seropositive Urban Men's Intervention Trial (SUMIT, N = 1168). These studies were conducted from 1996 to 2002 with racially diverse samples from New York and San Francisco. Patterns of sexual behavior often reflected an understanding of the relative risks of specific sexual practices and were generally consistent with harm reduction strategies to reduce the risk of HIV transmission to uninfected partners. Some men, however, continued to engage in behaviors that placed themselves and their partners at risk for exposure to HIV and other sexually transmitted infections. Correlates of unprotected sex included self-efficacy, personal responsibility, substance use, mental health, and contextual influences. Disclosure of HIV status was a difficult issue for many HIV-positive MSM. Most participants had disclosed to their main partner, but they disclosed to less than half of their non-main partners before first sex. The interest of HIV-positive MSM in prevention efforts, the design of the SUMIT intervention trial, and implications for future research and programmatic efforts are discussed.