Self-assessed health status has been shown to be a powerful predictor of mortality, service use, and total cost of medical care treatment. We investigated the potential for self-assessed health to further serve as a predictor of improvement in health status after a clinical intervention. Using the five-category measure of self-assessed health (excellent, very good, good, fair, or poor), we examined patients' improvements in health status after total knee arthroplasty in each of the WOMAC-defined categories for health status in patients. The results indicate that the greater patients rated their preoperative health, the greater their postoperative improvement. The results suggest that a simple process of asking patients to rate their own health in a presurgery clinic could be a powerful tool in predicting patient outcome. This also suggests that by stratifying preoperative self-assessed health, potential improvements in health status will be more fully captured.