Dialectical behaviour therapy (DBT) is widely considered to be a promising treatment for borderline personality disorder (BPD). However, the evidence for its efficacy published thus far should be regarded as preliminary. To compare the effectiveness of DBT with treatment as usual for patients with BPD and to examine the impact of baseline severity on effectiveness. Fifty-eight women with BPD were randomly assigned to either 12 months of DBT or usual treatment in a randomised controlled study. Participants were recruited through clinical referrals from both addiction treatment and psychiatric services. Outcome measures included treatment retention and the course of suicidal, self-mutilating and self-damaging impulsive behaviours. Dialectical behaviour therapy resulted in better retention rates and greater reductions of self-mutilating and self-damaging impulsive behaviours compared with usual treatment, especially among those with a history of frequent self-mutilation. Dialectical behaviour therapy is superior to usual treatment in reducing high-risk behaviours in patients with BPD.