THE STUDY GROUP CONSISTED OF PATIENTS SEEN AT A STATE PSYCHIATRIC HOSPITAL OR INDIVIDUALS REFERRED FOR A PSYCHIATRIC OPINION. A MAJORITY OF SUBJECTS AGED 40 YEARS AND OVER WERE CRIMINALLY INSANE. MOREOVER, A POSITIVE FAMILY HISTORY OF PSYCHIATRIC ILLNESS WAS PRESENT IN A MAJORITY OF THE CRIMINALLY INSANE GROUP. MURDER AND ATTEMPTED MURDER IN THE SETTING OF A STABLE MARRIAGE WERE ALMOST INVARIABLY THE RESULT OF SERIOUS PSYCHIATRIC ILLNESS. CASES OF SCHIZOPHRENIA AND DEPRESSIVE ILLNESS WERE SEPARATED FROM BOTH THE MIXED PARANOID GROUP AND A LARGE GROUP OF INDIVIDUALS WITH SEVERE PERSONALITY DISORDERS. THE ARTICLE MAKES A CASE FOR THE WIDER USE OF 'UNFITNESS TO PLEAD' IN THE SEVERELY PSYCHOTIC. FOLLOWUP OF THE CASES DISCOVERED THAT SUBJECTS FOUND NOT GUILTY ON PSYCHIATRIC GROUNDS WERE LIKELY TO BE HELD FOR LONGER PERIODS OF TIME THAN THOSE TREATED AS CRIMINALS. IT IS SUGGESTED THAT THE PUBLIC WOULD BE BETTER PROTECTED IF THE CROWN SOUGHT AN INSANITY VERDICT IN SUCH CASES RATHER THAN OPPOSE IT. THE EMPHASIS OF THIS PAPER IS CLINICAL, AND THE VARIED MODE OF COLLECTION OF THE CASES AND THEIR SMALL NUMBER ALLOW NO CLAIM FOR STATISTICAL SIGNIFICANCE. TABULAR DATA AND REFERENCES ARE INCLUDED. (AUTHOR ABSTRACT MODIFIED--MJW)