Deliberate self-harm (DSH) presents a significant health problem, especially as treatments have not been particularly successful in reducing repetition. Dialectical behaviour therapy (DBT; Linehan, 1993) is one approach that has reported some success in reducing self-harm rates in borderline personality disorder patients, who self-harm frequently, though it remains largely untested outside its original setting. The present study aimed to assess the effectiveness of DBT in self-harming women in an institutional setting in the United Kingdom where self-harm is common. Female patients at Rampton Hospital who were displaying self-harming behaviour and met criteria for borderline personality disorder (N = 10) participated in the full one-year treatment package of DBT. Patients were assessed on self-harm rates and on a number of psychological variables, pre-, during- and post-therapy, including a 6-month follow-up. There was a significant reduction in DSH during therapy, which was maintained at 6-month follow-up. This was paralleled by a reduction in dissociative experiences and an increase in survival and coping beliefs, alongside improvements in depression, suicide ideation, and impulsiveness. The findings are preliminary but the results suggest that DBT might provide an effective treatment for severe self-harm in institutional settings, and also highlight some of the psychological mechanisms that might mediate these improvements in self-harming behaviour.