Understanding the determinants of individuals' perceptions of their risk of becoming infected with HIV and their perceptions of acceptable strategies of prevention is an essential step towards curtailing the spread of this disease. We focus in this paper on learning and decision-making about AIDS in the context of high uncertainty about the disease and appropriate behavioral responses, and we argue that social interaction is animportant determinant of risk perceptions and the acceptability of behavioral change. Using longitudinal survey data from rural Kenya and Malawi, we test this hypothesis. We investigate whether social interactions-and especially theextent to which social network partners perceive themselves to be at risk -exert causal influenceson respondents' risk perceptions and on one approach to prevention, spousal communication about the threat of AIDS to the couple and their children. The study explicitly allows for the possibility that important characteristics, such as unobserved preferences or community characteristics, determine not only the outcomes of interest but also the size and composition of networks. The most important empirical result is that social networks have significant and substantial effects on risk perception and the adoption of new behaviors even after controlling for unobserved factors.