We assessed parents' beliefs about treatment credibility and effectiveness and examined the influence of these beliefs on subsequent treatment participation. Seventy-six parents completed the Credibility/Expectancies Questionnaire-Parent Version (CEQ-P), and subsequently participated in treatment for their child's clinically referred conduct problems. The key findings were that: (a) the CEQ-P is composed of two components that measure parents' treatment credibility and expectancies; (b) the total scale and each component are internally consistent and have strong test-retest reliability; (c) scores on the CEQ-P are significantly associated with scores on a measure of parent motivation for treatment, supporting the construct validity of this measure; and (d) scores on the CEQ-P at the first clinic visit significantly predict subsequent adherence to treatment procedures above and beyond demographic variables and parent motivation for treatment. This study provides an efficient and psychometrically sound measure of parent beliefs about treatment and demonstrates the importance of such beliefs for subsequent treatment adherence.