This study examined medical interpreters' practice of the co-diagnostician role and further explored its practical, institutional, and ethical implications. Twenty-six professional interpreters (of 17 languages), 4 patients, and 12 health-care providers were recruited for this study, which involves participant observation and interviews undertaken in the Midwestern US. Constant comparative analysis was used to develop themes of interpreters' communicative practices. Interpreters justified their role performances by claiming the identity of a member of the health care team and their work as part of the team effort. Their communicative strategies as a co-diagnostician reflect their preconception of the social hierarchy of health-care settings and the emphasis on diagnostic efficacy. I have identified five strategies for the co-diagnostician role. These were assuming the provider's communicative goals; editorializing information for medical emphasis; initiating information-seeking behaviors; participating in diagnostic tasks; and volunteering medical information to the patients. Although many strategies can be attributed to interpreters' effort to conserve providers' time and to bridge the cultural differences, they also pose risks to patients' privacy, clinical consequences, and provider-patient relationships.