Evidence suggests that a moderate amount of variance in patient outcomes is attributable to therapist differences. However, explained variance estimates vary widely, perhaps because some therapists achieve greater success with certain kinds of patients. This study assessed the amount of variance in across-session change in symptom intensity scores explained by therapist differences in a large naturalistic data set (1,198 patients and 60 therapists, who each treated 10-77 of the patients). Results indicated that approximately 8% of the total variance and approximately 17% of the variance in rates of patient improvement could be attributed to the therapists. Cross-validation and extreme group analyses validated the existence of these therapist effects.