This study examined sources of therapist effects in a sample of 25 therapists who saw 1,141 clients at a university counseling center. Clients completed the Outcome Questionnaire-45 (OQ-45) at each session. Therapists' facilitative interpersonal skills (FIS) were assessed with a performance task that measures therapists' interpersonal skills by rating therapist responses to video simulations of challenging client-therapist interactions. Therapists completed the Social Skills Inventory (SSI) and therapist demographic data (e.g., age, theoretical orientation) were available. To test for the presence of therapist effects and to examine the source(s) of these effects, data were analyzed with multilevel modeling. Of demographic predictor variables, only age accounted for therapist effects. The analysis with age, FIS, and SSI as predictors indicated that only FIS accounted for variance in outcomes suggesting that a portion of the variance in outcome between therapists is due to their ability to handle interpersonally challenging encounters with clients.