The relationship between therapists and treatment outcome was examined in 14 highly trained therapists who participated in the Multicenter Collaborative Study for the Treatment of Panic Disorder. Overall, therapists yielded positive outcomes in their caseloads; yet, therapists significantly differed in the magnitude of change among caseloads. Effect sizes for therapist impact on outcome measures varied from 0% to 18%. Overall experience in conducting psychotherapy was related to outcome on some measures, whereas age, gender, gender match, and experience with cognitive-behavioral therapy (CBT) were not. Therapists with above- and below-average outcomes were rated similarly on measures of adherence and competency. The results suggest that therapists make a contribution to outcome in CBT for panic disorder, even when patients are relatively uniform, treatment is structured, and outcome is positive. Implications for future clinical outcome studies and for training clinicians are discussed.