Negative therapeutic reaction was first described by Freud, and theories about this phenomenon have focused on various patient factors, for example, unconscious guilt, narcissistic pride, and overwhelming feelings of shame, as well as devaluation of the therapist as a defense against envy. Different strategies to counteract negative therapeutic reactions in patients with severe personality disorder have been proposed. Bateman and Fonagy suggested that some psychoanalytic techniques are iatrogenic for patients with borderline personality disorder and, thus, the therapeutic method must be modified. One manifestation of negative therapeutic reaction is that patients discontinue therapy prematurely. The aim of this study was to explore the differences in the early therapeutic process between therapy dropouts and therapy completers in mentalization-based treatment (MBT) for patients with concurrent borderline personality disorder (BPD) and substance use disorder (SUD). The first 2 videotaped therapy sessions from 6 MBT therapies were selected—3 therapies that were completed and 3 therapies that were prematurely discontinued by the patient. The therapy process was analyzed using the Psychotherapy Process Q set (PQS). Lists of the most frequent and least frequent PQS items among therapy completers versus dropouts are presented. In comparing the sessions from therapy completers and dropouts, significant differences in frequencies were detected for 13 PQS items. These items included therapist and patient activities, as well as features of the therapeutic interaction. Continuation in therapy seems to be related to therapists’ general skills and adherence to MBT, as well as patients bringing up significant material and not being distant.