Population segments at highest risk for HIV are often hidden, marginalized, and hard to reach by conventional prevention programmes. This pattern is especially true in Central and Eastern Europe, where major HIV epidemics have recently appeared, where population members do not perceive themselves as belonging to a community, and where there is little precedence for strong community-based organization service programmes. In these circumstances, naturally existing intact social networks still can be targeted by prevention programmes. HIV prevention interventions undertaken with at-risk social networks can establish new group norms, reduce the risk behaviour of network members, and can reach 'hidden' members of a population known personally to leaders of the social networks. This article illustrates a methodology and a practical description for: (1) accessing high-risk social networks in a community population; (2) identifying and enumerating the membership of the social networks; (3) identifying the social leadership of the networks; and (4) establishing the HIV risk behaviour levels of the recruited networks. To illustrate how social network methods can be applied in the field, the article provides case study reports of HIV prevention fieldwork practice targeting high-risk networks of young men who have sex with men and young heterosexual adults in St Petersburg, Russia. Although there is an extensive conceptual literature on the influence of social networks on risk behaviour, this article describes specific and practical techniques that can be in the development of approaches for social network-based interventions.