Randomized controlled trials have shown that using client feedback obtained from routine outcome monitoring can lead to better clinical outcomes in psychotherapy. However, existing randomized controlled trial studies did not address the between-person and within-person effects of routine outcome monitoring simultaneously. To fill this critical gap, this study conducted a multilevel analysis to explore the effect of therapist utilization of client symptom outcome data in a naturalistic practice setting. Participants were 26 therapist trainees and their 456 clients who conducted 5,917 sessions in total. Clients completed the Outcome Questionnaire (OQ)-45.2 prior to every session, and we used de-identified data to code whether each OQ report was reviewed by the therapist