Diagnoses are the classification tools of medicine, and are pivotal in the ways medicine exerts its role in society. Their sociological study is commonly subsumed under the rubrics of medicalisation, history of medicine and theory of disease. Diagnosis is, however, a powerful social tool, with unique features and impacts which deserve their own specific analysis. The process of diagnosis provides the framework within which medicine operates, punctuates the values which medicine espouses, and underlines the authoritative role of both medicine and the doctor. Diagnosis takes place at a salient juncture between illness and disease, patient and doctor, complaint and explanation. Despite calls for its establishment, almost two decades ago (Brown 1990), there is not yet a clear sociology of diagnosis. This paper argues that there should be, and, as a first step, draws together a number of threads of medical sociology that potentially contribute to this proposed sociology of diagnosis, including the place of diagnosis in the institution of medicine, the social framing of disease definitions, the means by which diagnosis confers authority to medicine, and how that authority is challenged. Through this preliminary review, I encourage sociology to consider the specific role of diagnosis in view of establishing a specific sub-disciplinary field.