The authors studied the relationship between DSM-III axis I and axis II diagnoses in 2,462 medical center patients. Personality disorders were most commonly associated with substance use disorders and with the anxiety and somatoform disorders traditionally classified as neuroses. There was a particularly strong connection between antisocial personality disorder and substance abuse. The psychotic and major affective illnesses were significantly less often associated with personality disorders. The results of this study suggest a need for refining the criteria for several DSM-III categories. Overall, the separate personality disorder axis in the DSM-III system provides information not contained with the syndromal classification alone.