UNLABELLED: Psychologic status is associated with poor outcome after knee arthroplasty yet little is known about which specific psychologic disorders or pain-related beliefs contribute to poor outcome. To enhance the therapeutic effect of a psychologic intervention, the specific disorders or pain-related beliefs that contributed to poor outcome should be identified. We therefore determined whether specific psychologic disorders (ie, depression, generalized anxiety disorder, panic disorder) or health-related beliefs (ie, self-efficacy, pain catastrophizing, fear of movement) are associated with poor outcome after knee arthroplasty. We conducted a cohort study of 140 patients undergoing knee arthroplasty at two hospitals. Patients completed a series of psychologic measures, provided various sociodemographic data, and were followed for 6 months. Patients were dichotomized to groups with either a favorable or a poor outcome using WOMAC pain and function scores and evidence-based approaches. After adjusting for confounding variables, we found pain catastrophizing was the only consistent psychologic predictor of poor WOMAC pain outcome. No psychologic predictors were associated consistently with poor WOMAC function outcome. An intervention focusing on pain catastrophizing seems to have potential for improving pain outcome in patients prone to catastrophizing pain. LEVEL OF EVIDENCE: Level I, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.