To determine the justification of a continuous passive motion machine in the treatment of postoperative total knee arthroplasties, a comparative study of 50 consecutive patients with simultaneous bilateral total knee arthroplasties was undertaken. The patients served as their own controls because one randomly selected knee was placed in the machine while the remaining knee was treated with physical therapy only. There was no significant difference in the range of motion during the eight days of hospitalization or the follow-up visits at two weeks, two months, six months, and one year. There was a significant decrease in the swelling about the knee. The continuous passive motion treated knees appeared to be generally weaker as revealed by more extensor lags and flexor tightness at discharge from the hospital. Also, increased costs incurred from the need for additional equipment and increased staff time made the machine neither cost-effective nor beneficial.