Aims: There is a scarcity of clinical trials on psychological treatments for concurrent borderline personality disorder (BPD) and substance use disorder (SUD). Mentalization-based treatment (MBT) have shown efficacy in several trials on BPD. The aim of the present study was to examine the feasibility and effectiveness of MBT for concurrent BPD and SUD. Methods: Patients (n = 46) with concurrent BPD and SUD were randomized either to MBT in combination with SUD treatment (n = 24) or to SUD treatment alone (n = 22). Outcome was measured after 18 months using objective data, as well as interview and self-report measures. Results: There was no significant difference between the groups on any outcome variable. No suicide attempts occurred in the MBT group in contrast to 4 suicide attempts that occurred in the control group - a difference that did not reach statistical significance (p = 0.06). A majority of the therapists did not show sufficient MBT adherence and quality. Conclusion: MBT for patients with concurrent BPD and SD does not appear to be harmful; on the other hand, it is possibly helpful in reducing the risk involved in suicide attempts.