Background and Purpose. Physical and psychosocial factors are hypothesized to influence performance-based assessment. The purpose of this study was to evaluate the association between performance on the Isernhagen Work System Functional Capacity Evaluation (IWS-FCE) and various clinical and psychosocial factors. Subjects. The sample consisted of 170 workers' compensation claimants who were undergoing functional capacity evaluations (FCEs) for low back injuries. Methods. In this cross-sectional study, claimants completed a battery of work-related measures, including the IWS-FCE, the Pain Disability Index (PDI), a workplace organizational policies and practices scale, and a recovery expectations questionnaire. Functional capacity evaluation performance indicators were the number of tasks in which subjects did not meet work demands and weight lifted on the floor-to-waist lift. Analysis included multivariable regression. Results. Only the PDI, pain intensity, age, and sex independently contributed to floor-to-waist lift performance. The PDI, pain intensity, and duration of injury contributed to the number of failed tasks. Discussion and Conclusion. The results indicate that performance on FCEs is influenced by physical factors, perceptions of disability, and pain intensity. However, perceptions of workplace organizational policies and procedures were not associated with FCE results for workers' compensation claimants with chronic back pain disability. Functional capacity evaluations should be considered behavioral tests influenced by multiple factors, including physical ability, beliefs, and perceptions.