This study aimed to evaluate the differential effect of outcome monitoring feedback to therapists and to patients on outcomes in cluster B, cluster C, and personality disorder not otherwise specified (PD-NOS) patients. Day treatment patients (n = 112) and inpatients (n = 94) were randomly assigned to a feedback to therapist (FbT), feedback to therapist and patient (FbTP), or no feedback (NFb) condition. Feedback was based on weekly administrations of the Outcome Questionnaire-45 (OQ-45). In cluster B and PD-NOS patients FbTP resulted in increased OQ-45 scores during the first 6 months of therapy for not on track (NOT) patients. In cluster C patients, no adverse effects of feedback were found. These results suggest that for certain personality disorder patient groups, providing feedback during treatment may not always be beneficial, although more research is needed to further assess these effects.