Introduction: Patients' initial beliefs about the success of a given pain treatment are shown to affect final treatment outcome. The Credibility/Expectancy Questionnaire (CEQ) has recently been developed as measure of treatment credibility and expectancy. Objective: The objectives of this study were (1) to investigate the factor structure of the CEQ in a sample of chronic low back pain (CLBP) patients by means of a confirmatory factor analysis, (2) to examine the association between treatment credibility and expectancy and patient characteristics, and (3) to assess whether treatment expectancy and credibility are associated with the outcome of rehabilitation treatment. Methods: CLBP patients (n = 167) were randomized to either active physical therapy (n = 51), cognitive-behavioral therapy (n = 57), or a combination therapy (n 59), and completed the CEQ after a careful explanation of the treatment rationale. Results: Confirmatory factor analysis supported the 2-factor structure (credibility/expectancy) of the CEQ. Lower credibility was associated with higher pain-related fear and lower internal control of pain, and lower expectancy with higher levels of pain-related fear and no radiating pain. Multiple linear regression analyses revealed that after controlling for age, sex, treatment center, pain-intensity at baseline, duration of disability, and irrespective of the treatment offered, expectancy was significantly associated with disability and satisfaction. Credibility was significantly associated with patient-specific symptoms and satisfaction. For global perceived effect, treatment expectancy was predictive in active physical therapy only, and treatment credibility was a significant predictor in combination therapy only. Discussion: Although the associations found were low to modest, these results underscore the importance of expectancy and credibility for the outcome of different active interventions for CLBP and might contribute to the development of more effective treatments.