We performed primary bilateral total knee replacement (T.K.R.) in twelve patients by one stage operation (group I ) and compared with ten patients who underwent bilateral T.K.R. by two stage operation (group II ) at Severance Hospital from January 1986 to December 1989. We have used Miller-Galante Knee Systems in all patients. They were followed for more than 1 year, and comparatively analysed retrospectively, and the results were as follows:1. Pre-operative postoperative knee rating scores were measured with the criteria of Hospital for Special Surgery, and changed from 50.6 into 84 in group I, and from 36.2 to 86 in group II. 2. The postoperative pain and range of motion were not significantly different in both groups. 3. The postoperative complications were reactivation of tuberculosis, deep seated infection, and patellar component loosening in group I, and urinary tract infection in group II Complications in group I were not related to stage of operation, but related to conditions of patients. 4. Comparing to two stage operation, the advantages of one stage operation were shorter anesthethia and operation time, lesser use of antibiotics, reduced hospital cost and hospitalization, shorter period of rehabilitation, and lesser use of blood transfusion. In conclusion, we recommend one stage, simultaneous bilateral total knee replacement, if the patient's condition allows prolonged anesthetic time, and if surgeons have familiarity with the technique.