Background Intensive care Unit (ICU) admission decisions involve collaboration between internists and intensivists. Clear perception of each other's roles is a prerequisite for good collaboration. The objective was to explore how internists and intensivists perceive their roles during admission decisions. Methods Individual in-depth interviews with 12 intensivists and 12 internists working at a Swiss teaching hospital. Interviews were analyzed using a thematic approach. Results Roles could be divided into practical roles and identity roles. Internist and intensivists had the same perception of each other's practical roles. Internists' practical roles were: recognizing signs of severity when the patient becomes acutely ill, calling the intensivist at the right moment, having the relevant information about the patient and having determined the goals of care. Intensivists' practical roles were: assessing the patient on the ward, giving expert advice, making quick decisions, managing access to the ICU, having the final decision power and, sometimes, deciding whether or not to limit treatment. In complex situations, perceived flaws in performing practical roles could create tensions between the doctors. Intensivists' identity roles included those of leader, gatekeeper, life-death decision maker, and supporting colleague doctors (consultant, senior and helper). These roles could be perceived as emotionally burdensome. Internists' identity roles were those of leader and partner. Conclusions Despite a common perception of each other's practical roles, tensions can arise between internists and intensivists in complex situations of ICU admission decisions. Training in communication skills and interprofessional education interventions aimed at a better understanding of each other roles would improve collaboration.