Personal interviews and examinations, and records searches formed the basis of this study. All findings were entered into a 229-item precoded questionnaire. This protocol provided a uniform record of relevant demographic, psychiatric, substance abuse, developmental, educational, marital, criminal, and military service information for each subject. Data analysis confirmed several important findings: (1) sexual diagnoses were found exclusively among sexual offenders, with rapists sharing two diagnoses, pedophilia and sexual sadism, while rapists/murderers demonstrated an extraordinarily high prevalence of sexual sadism; (2) substance abuse disorders were highly prevalent in all groups; and (3) psychotic psychopathology was most prevalent among murderers. This article was not meant to suggest that one can or should distinguish criminal offender types solely on the basis of diagnosed psychopathology. Many other characteristics require examination in order to illuminate the diversity of social, psychological, and biological factors that can contribute to criminal behavior. Tables, references