Different methods are available for the psychotherapeutic treatment of patients with severe personality disorders. In Germany, a special form of dynamically oriented therapy called psychoanalytic-interactional psychotherapy or method (PiM) has been clinically applied for many years. PiM was derived from psychoanalytic therapy and has been specifically adapted for the treatment of severely disordered patients, for example, patients with borderline personality disorders, prepsychotic disorders, addictions, and perversions. In a naturalistic study, the effectiveness of PiM was tested in a sample of patients with borderline personality disorders (N = 132). The patients were treated in the Clinic Tiefenbrunn near Göettingen. Standardized, reliable, and valid diagnostic instruments were used to study the treatment effects. According to the results, PiM achieved significant improvements in target symptoms, general symptoms, interpersonal problems, and contentedness with life. The results are discussed with regard to the treatment of severely disordered patients.