Faced with the recommendation by an outside consultant to construct a large hospital for the criminally insane, the Oklahoma legislature approved a proposal by the state mental health department to study the mental health treatment needs of the prison population. Through a variety of assessment techniques, the authors found the inmates had a spectrum of disorders needing different treatment approaches. Approximately 78 per cent of the inmates are diagnosable. Thirty-five per cent require treatment other than brief intervention, and 10 per cent are seriously ill. It is estimated that 20 per cent may be resistive and may require involuntary treatment.