Little is known about the effectiveness of treatments for severe personality disorder in prisons. Any treatment within the prison context will aim to reduce criminogenic risk in its participants, and ameliorate the overall symptomatology of the disorder. The over-representation of borderline personality disorder in female prisons has lead to pilots of dialectical behaviour therapy in three such establishments in the UK. This treatment was designed for borderline personality disordered women in the community in the USA, and this is the first time it has been piloted within a UK prison context. It was the treatment of choice as it had the strongest evidence base in lower security settings. Overall findings from the pilots have been very promising in terms of the viability of DBT as an offending behaviour programme and in terms of improving the manageability of prisoners on the wing. The case studies described here aim to illustrate in some detail the cognitive and behavioural change process in individuals over a year of treatment, and at six-month follow-up, and to highlight the particular challenges faced by participants and therapists when delivering treatment in prison. In describing the improvements in all three cases, which vary in terms of symptomatology and background, we aim to demonstrate the versatility of DBT in the prison setting and its capacity for reducing criminogenic risk. (PsycINFO Database Record (c) 2016 APA, all rights reserved)