The impact of the counselor in substance abuse rehabilitation has been questioned. The unexpected resignations of two counselors provided a natural opportunity to examine the effects of counselor assignment in a methadone maintenance treatment program with the effects of the medication philosophy, program rules, and supplementary services held constant. Sixty-one patients who had been assigned to these two counselors were assigned to four other counselors in a virtually random manner. It was reasoned that if the new counselor had relatively little impact, then there would be little difference in performance from pre to posttransfer or among the four caseloads during the 6-month period following the transfer. Performance measures included urinalysis results, methadone dosage, prescriptions for psychotropic medications, employment, and arrest rates. Results indicated statistically significant and clinically meaningful differences in the posttransfer performance of the four caseloads. One counselor significantly reduced the average methadone dose of his/her caseload as well as the number of patients prescribed ancillary medications, while concurrently reducing positive urine tests, unemployment and arrests. In contrast, another counselor significantly increased the average methadone dose in his/her caseload but still showed increases in positive urine tests and unemployment. As has been found in prior studies, background and formal education differences among the counselors were not related to the observed performance differences. However, differences in the content and process of counseling among the counselors were associated with the differences in patient outcome. These process differences are discussed in relation to earlier studies of professional psychotherapy.