Patients' initial beliefs about the success of a given pain treatment are shown to have an important influence on the final treatment outcome. The aims of the paper are to assess determinants of patients' treatment expectancy and to examine the extent to which treatment expectancy predicts the short-term and long-term outcome of cognitive-behavioral treatment of chronic pain. This study employs the data of 2 pooled randomized clinical trials evaluating the effectiveness of cognitive-behavioral interventions for 171 patients with fibromyalgia and chronic low back pain. Pretreatment and posttreatment expectancy were measured by a short questionnaire, which was based on the procedure by Borkovec and Nau. Four composite outcome variables (pain coping and control, motoric behavior, negative affect, and quality of life) were measured before and after the intervention and at 12 months follow-up. Furthermore, several patient characteristics were taken into account. Patients with higher treatment expectancies significantly received less disability compensation and were less fearful. A regression model of 3 factors (better pain coping and control, active and positive interpretation of pain, and less disability compensation) significantly explained 10% of the variance in pretreatment expectancy. Pretreatment expectancy significantly predicted each of the 4 outcome measures immediately after treatment and at 12 months follow-up. This study corroborates the importance of treatment expectation before entering a cognitive-behavioral intervention in patients with chronic musculoskeletal pain.