At Risk Mental State (ARMS) clinics are specialised mental health services for young, help-seeking people, thought to be at ultra-high risk of developing psychosis. Their stated purpose is to reduce transitions from the ARMS state to clinical psychotic disorder. Reports of ARMS clinics provide ‘evidence-based recommendations’ or ‘guidance’ for the treatment of such individuals, and claim that such clinics prevent the development of psychosis. However, we note that in an area with a very well-developed ARMS clinic (South London), only a very small proportion (4%) of patients with first episode psychosis had previously been seen at this clinic with symptoms of the ARMS. We conclude that the task of reaching sufficient people to make a major contribution to the prevention of psychosis is beyond the power of ARMS clinics. Following the preventative approaches used for many medical disorders (e.g. lung cancer, coronary artery disease), we consider that a more effective way of preventing psychosis will be to adopt a public health approach; this should attempt to decrease exposure to environmental factors such as cannabis use which are known to increase risk of the disorder.