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 toThe IDEA group comprises, in alphabetical order: . Conducting research in such high secure settings places significant burdens on all involved and we were received with cooperation and generosity. The opinions expressed in this paper are those of the authors and not necessarily those of the Ministry of Justice.