Hospital tumor boards bring together multidisciplinary specialists to devise the best treatment program for cancer patients. However, research has found that tumor board decisions do not always represent the multispecialty perspectives of their participants. Previous studies have emphasized barriers to tumor board effectiveness such as a lack of positive leadership, administrative support, adequate information and a standardized process, but few studies have examined how tumor board culture and structure influences the treatment decision-making process. Sociological theories suggest that while group processes can improve decision-making, invisible status hierarchies may stymie collaboration. Through ethnographic exploration and theoretical application of 44 hospital tumor boards in the United States and the United Kingdom, this study shows how social hierarchical structures in tumor boards influence treatment decision-making processes. The findings change how we see tumor boards from an unbiased clinical organization to one influenced by social forces of hierarchical structures.