Background: There is a high volume of unmet needs for knee arthroplasty in the population despite the increase in surgery rates. Given the long waiting times to have a knee arthroplasty, some governments have proposed prioritization systems for patients on waiting lists based on their level of need. Questions/Purposes: We therefore estimated the needs and demand of knee arthroplasty in four regions of Spain during a 5-year period. Methods: We developed a discrete event simulation model to reproduce the process of knee arthroplasty. The prioritization system was compared with the usual waiting list management strategy (by waiting time only). Results: Under the prioritization system, patients saved an average of 4.5 months (95% confidence interval, 4.4-4.6 months) adjusted by level of need. The proportion of patients who experienced excessive waiting times was small and was associated with low levels of priority. The 5-year projection of the volume of unmet needs for knee arthroplasty remained stable; however, although the volume of need for the first knee arthroplasty decreased by 12%, the volume of need for an arthroplasty in the contralateral knee increased by 50%. Conclusions: The data suggested the prioritization system was more beneficial than assigning surgery by waiting time only. The 5-year projection of the volume of unmet needs for knee arthroplasty remained stable, despite the increase in the need for contralateral knee arthroplasty. Level of Evidence: Level II, economic and decision analyses. See Guidelines for Authors for a complete description of levels of evidence. © 2009 The Association of Bone and Joint Surgeons®.