ME/CFS is a medically contested illness and its understanding, framing and treatment has been the subject of heated debate. This paper specifically examines why framing the condition as a psychiatric issue-what we refer to as 'psychiatrisation'-has been so heavily contested by patients and activists. We argue that this contestation isn't simply about stigmatising mental health conditions, as some have suggested, but relates to how people diagnosed with mental illness are treated in society, psychiatry and the law. We highlight the potentially harmful consequences of psychiatrisation which can lead to people's experiential knowledge being discredited. This stems, in part, from a psychiatric-specific form of epistemic injustice which can result in unhelpful, unwanted and forced treatments. This understanding helps explain why the psychiatrisation of ME/CFS has become the focus of such bitter debate and why psychiatry itself has become such a significant 'field of contention', for both ME/CFS patients and mental health service users/survivors. Notwithstanding important differences, both reject the way psychiatry denies patient explanations and understandings, and therefore share a collective struggle for justice and legitimation. Reasons why this shared struggle has not resulted in alliances between ME and mental health activists are noted.