BACKGROUND Recent research on the HIV epidemic in sub-Saharan Africa has highlighted the relevance of married individuals' extramarital sexual behavior for the spread of the disease. At the same time, there is social disapproval of sexual infidelity. OBJECTIVE This article examines the extent to which Malawian married men's likelihood of having extramarital sex is influenced by their expectations about the prevalence of extramarital relationships in their social network. It also explores whether this effect depends on the network density, and whether it is also observed when the extramarital behavior of a particularly influential actor is controlled for. METHODS Data from the last two waves, 2004 and 2006, of the longitudinal survey provided by the Malawi Diffusion and Ideational Change Project are analyzed both cross-sectionally and through a panel analysis with fixed effects. The longitudinal approach enables the researcher to deal with the potential non-random distribution of social interactions among respondents, which bias the estimation in the cross-sectional analysis. RESULTS Married men's expectations about the prevalence of extramarital sexual relationships in the network were shown to have a substantial influence on their extramarital behavior, and the impact was found to be bigger in dense networks. In addition, there was some evidence that the perceived dominant behavior in the peer group is relevant, independent of the extramarital behavior of the respondents' best friends.