The objective of this study was to determine whether treatment outcome expectancy, treatment credibility, locus of control, and fear avoidance beliefs predict treatment success of manual therapy and physical therapy for patients with non-specific neck pain. This study is a secondary analysis of a randomized controlled trial conducted in primary care practices in The Netherlands. It was found that outcome expectancy in patients with non-specific neck pain has additional predictive value for treatment success above and beyond clinical and demographic variables.