While there has been a veritable boom in literature on organized interests, their lobbying strategies, relationships with decision-makers, and their impact on policymaking, only a few studies have explored internal organizational developments and, specifically, the professionalization of interest groups. The present study focuses on the national and transnational factors driving the professionalization of interest groups in Central and Eastern Europe, a region previously neglected in much of the interest group literature. Based on a sample of more than 400 surveyed organizations operating in Poland, the Czech Republic, Hungary, and Slovenia in the healthcare, higher education, and energy sectors, we explore three bundles of factors potentially enhancing the professionalization of interest groups – organizational funding sources, national and transnational intergroup cooperation and organizations’ standing in the domestic interest group system. Our statistical analyses show that state subsidies and tight policy coordination with the state are crucial drivers of internal organizational professionalization, suggesting rather patronistic and symbiotic relationships between the state and certain organizations. However, our data also support the notion that interorganizational collaboration, both at the national and international levels, may also be key to organizational professionalization, enabling groups that lack close ties with the state to compensate their disadvantage with intensive domestic and international networking. The study is also among the first to link increasing professionalization with organizational population density.