In most interest group systems, only a small number of organized interests demonstrate a high level of political activity. Yet, while a diverse set of groups mobilizes politically and often enjoys government access, they are unlikely to be considered equally influential. Therefore, a key question in interest group politics involves how organizational features, interorganizational networks and patterns of institutional access shape interest group influence. To address this question, we zoom in on organized interests within one particular policy domain, namely healthcare in Belgium, and assess the social mechanisms that produce the varying influence reputation of interest groups. Our analyses rely on a social network analysis of evidence collected through structured interviews with health policy experts as well as 45 prominent health advocacy groups.