This article examines how networks of social and sexual relations affect risky sexual behavior and HIV seroprevalence among young gay men. Social networks can transmit information and cultural norms regarding safer sex, while networks of sexual partners channel the risk of exposure to HIV infection. These two network effects may help to explain some of the behavior and seroconversion differentials in the gay community. A number of recent studies have shown higher rates of unsafe sex among younger gay men. In the Longitudinal AIDS Impact Project, for example, younger gay men (18-24) report unsafe receptive anal sex at rates double that for any other age group (30% vs. 14-16%). One possible explanation is that younger men have watched fewer friends and colleagues contract HIV or AIDS, and are correspondingly less cautious. We test this hypothesis by comparing the personal networks of younger and older gay men to see whether those who practice safer sex have more exposure to persons with HIV or AIDS. The results give only weak support for the hypothesis that personal exposure to the effects of HIV and AIDS increases adherence to safer sex practices. Seroprevalence patterns among young men may be the result of their sexual networks, with those choosing older partners more likely to be exposed to HIV infection. We examine this hypothesis by comparing the age composition of the unsafe sexual partner network for seropositive and seronegative young men. The results strongly support the hypothesis that younger gay men with older partners are the leading edge of the epidemic in their cohort.