Background. Intensive Short-Term Dynamic Psychotherapy (ISTDP), as developed by Habib Davanloo, is an intensive emotion-focused psychodynamic therapy with an explicit focus on handling resistance in treatment. A core assumption in ISTDP is that psychotherapeutic effects are dependent on in-session emotional processing in the form of rise in complex transference feelings that occurs when treatment resistance is challenged. Recent research indicates that an unlocking of the unconscious, a powerful emotional breakthrough achieved at a high rise in complex transference feelings, can potentially enhance ISTDP’s effectiveness. While ISTDP has a growing evidence base, most of the research conducted has used small samples and has tested therapy delivered by expert therapists. The aims of this study were to evaluate the overall effectiveness of ISTDP when delivered in a tertiary psychotherapy service, and to investigate if having an unlocking of the unconscious during therapy predicted enhanced treatment effectiveness.