Diagnosis lies at the heart of the medical encounter but has received much less attention than treatment. It is widely assumed that negligent diagnosis claims should be governed by the Bolam test, but we demonstrate that this is not always the case. First, we disaggregate the diagnostic process into three different acts: forming the diagnosis, communicating it to the patient, and recording it. Second, we consider alternatives to Bolam for defining negligence, including less deferential profession-led standards, patient-led standards, and even a reasonable person standard. Third, bringing together these distinctions—within the diagnostic process, and between standards of care—we reveal the unappreciated complexity of negligent diagnosis. Analysing the standard of care that might apply to the three different acts in the diagnostic process, we identify reasons to think that Montgomery should apply to the communication of a diagnosis. We also argue that even in areas where the law is well-established, such as the application of Bolam to the formation of a diagnosis, challenging questions arise that require further attention. Throughout, the framework and analysis that we develop have significant implications for the outcomes in certain negligence cases, as well as for medical education, clinical guidelines, and patient care.