Therapist treatment adherence has received a great deal of attention in recent years, in part because of its relation to treatment outcomes. Although certain therapist behaviors have been found to be associated with treatment outcomes, little is known about client factors impacting on therapists’ ability to adhere to treatment protocols. In this study, we evaluated effects of parental beliefs, psychopathology, and interaction styles on therapists’ adherence to Multi-systemic Therapy (MST) principles. Eighty-two parents participating in a clinical trial of MST completed baseline measures assessing psychopathology, family functioning, and treatment expectations. Analyses indicated that parental perceptions of therapist adherence were established within the first 4 weeks of treatment, and that parental psychopathology, motivation, expectations, and child rearing practices were related to parental ratings of therapist adherence. Results were essentially unchanged when controlling for parental positive response style. Clinical and research implications of the findings are discussed.