Objective. Outcome monitoring feedback has become popular, but its effect on treatment outcome has been mixed. Feedback seems most effective for patients who are not progressing well (“not on track” (NOT) cases). There are some indications that patient feedback has an additional effect and that feedback effects differentiate between short- and long-term therapy. This study aimed to evaluate the effect of outcome monitoring feedback to therapists and patients on outcome in short- and long-term psychotherapy. Methods. Patients (n = 475) were randomly assigned to three conditions: Feedback to therapist (FbT), feedback to therapist and patient (FbTP), and no feedback (NFb). Feedback consisted of progress charts based on the Outcome Questionnaire and a feedback message. Results. In short-term therapies (<35 weeks) FbT and FbTP was preventive of negative change for NOT cases. In long-term therapy only FbTP had a small positive effect on the rate of change. Feedback did not result in better outcomes at treatment ending, although there was a trend for FbTP to have fewer deteriorated cases. Conclusions. Benefits of feedback were strongest for cases that were not progressing well in short-term therapies when both the patient and therapist received feedback on the patients' progress. Contrary to previous findings, we also found a small effect of feedback to therapists and patients in long-term therapies. Feedback to both patients and therapists may be more effective than feedback to therapists alone due to implementation issues or empowerment of the patient.