In this article, we summarize the results of a pilot study that investigated the effects of mentalization based treatment (MBT) for female patients with severe personality disorder (PD) and substance use disorder (SUD). We specially emphasize the group component (MBT-G) of the treatment. We describe the principles of MBT-G, the formidable challenges with this patient population, and how the therapists and the MBT team as a whole coped with these challenges. The overall treatment results were very good, both with respect to drug consumption and personality functioning. For most of the patients an extremely destructive lifestyle was profoundly changed.