England and Wales introduced the administrative category of Dangerous Severe Personality Disorder (DSPD) and established special units in prisons and high-secure psychiatric hospitals for their treatment. To examine their characteristics, we approached 202 patients admitted to DSPD units; 174 consented to participate in research. All were male, and the median age was 38 years. Most were white and born in the UK. Most patients (75%) met full DSPD criteria as suffering from severe personality disorder that caused them to be dangerous. With respect to personality disorder, most had elevated psychopathic traits and 40% met criteria for psychopathy on the Psychopathy Checklist-Revised (Hare, 2003). With respect to dangerousness, 25% of patients had a history of homicide; 28%, sexual offenses; and 41%, other violent offenses. Median time spent in prison was more than 12 years. According to standardized measures of violence risk, those admitted to DSPD units resembled other groups of high risk offenders described in the international literature. There were some significant differences between those admitted to prison- versus hospital-based units. The findings confirmed that DSPD units contain dangerous offenders with no evidence of preemptive incarceration.