 Standard one-year dialectical behaviour therapy DBT, which has four components, is an effective treatment for people with borderline personality disorder. However, such DBT programs are in short supply and costly, resulting in long wait lists. In practice, DBT is often reduced in length or intensity. This study will determine whether shorter DBT treatment is clinically effective and cost-effective. In total, 240 self-harming BPD patients will be randomly assigned to receive either 1 year or 6 months of DBT, with follow-up lasting two years. Rates of suicidal and self-harm behaviours, use of health care and general psychological functioning will be examined. 