BACKGROUND: As the numbers of HIV-positive diagnoses rise in South Africa, it is important to understand the determinants and consequences of HIV disclosure. METHODS: Cross-sectional survey from random community samples of men and women in urban and rural South Africa (n = 217 HIV-positive individuals, 89% female). RESULTS: Two thirds of all known HIV-infected adults in these communities had disclosed their status to sexual partner(s). On average, individuals who disclosed were 2 years older, higher in socioeconomic assets, and had known their HIV status 7 months longer than those who had not told their sexual partner(s). The "need for privacy" was the most cited reason (45%) for nondisclosure among those who had never disclosed. People who eventually disclosed their HIV status to sexual partner(s) were significantly more likely to report always or more frequently using condoms, reducing their number of sexual partners, and/or becoming monogamous. Among individuals who disclosed their HIV status, 77% reported increases in social support, with families providing the most support. CONCLUSIONS: Disclosure is associated with reports of consequent safer sexual behavior and greater social support. Interventions might be informed by the costs and benefits of disclosure and differences in disclosure to sexual partner vs. to one's social network.