The frequency of aggressive behaviour before and during admission to a hospital unit designed to treat patients identified as dangerous and as having a severe personality disorder (DSPD) was examined. Accounting for differences in recording procedures and time at risk, results showed a comparable frequency of aggressive behaviour across environments. Although the limited number of patients available to study and methodological issues prevent definitive conclusions, reactions to admission and exposure to the therapeutic context appeared inconsistent, some patients demonstrating stability or improvement, others appearing to worsen or react negatively to admission. Neither psychopathy nor severity of personality disorder interacted with admission and exposure to the new environment to impact on the course of aggressive behaviour. Results emphasise the need for staff to prepare for the inevitability of aggression in this group of patients during the lengthy, demanding and complex treatment process, and to consider the potential for a various reactions to admission, including deterioration, which may mean an increase in dangerousness or an aggravation of the personality disorder.