Behavioural psychotherapy has been demonstrated as the treatment of choice in most forms of serious mental disorders. In some instances it interacts synergistically with drug therapies. The Buckingham Project is a field trial of a comprehensive mental health service that employs behavioural analysis and therapy in the management of all cases of adult disorders occurring in an epidemiologically defined population. A multi-disciplinary team provide in vivo treatment. Results suggest that the incidence and prevalence of major episodes of mental illness have been substantially reduced and that the need for traditional hospital-based care has been minimised. The significance of this project for planning health services and for the future of behavioural psychotherapy is discussed.