In this study, the “sudden murderer” is defined as a person who, without having been involved in any previous serious aggressive antisocial acts, suddenly, unlawfully, and intentionally kills (or makes a serious attempt to kill) another human being. The murder is “sudden” in the sense that it appears to be a single, isolated, unexpected episode of violent, impulsive acting-out behavior—behavior which is never well thought out, behavior which has no obvious purpose or hope for personal advantage or profit foreseeable as a result. Of 153 criminal offenders referred for psychiatric evaluation between July 1, 1956, and Dec. 30, 1957, to the Social Maladjustment Study Unit at the Malcolm Bliss Mental Health Center in St Louis,* 13 could be called “sudden murderers” by this definition. Although in this group of patients the crime as a function of the personality of the patient concerned proved most