The incidence of complications, the need for secondary surgical procedures, and the mortality rate associated with bilateral replacement of the knee performed simultaneously, performed during the same hospitalization, or performed during separate hospitalizations were compared with those after unilateral replacement of the knee. The incidence of complications after 290 simultaneous bilateral procedures was 9.3 per cent, which compares favorably both with the incidence of 7.0 per cent after 228 bilateral procedures that were done during the same hospitalization and incidence of the 12.0 per cent after 234 bilateral procedures that were performed during separate hospitalizations. The incidence for each of these groups compares favorably with the incidence of complications of 11.0 per cent after 501 unilateral procedures. The incidence of reoperation was 2.4, 4.8, 8.5, and 5.6 per cent, respectively, in the four groups, and the incidence of mortality was 5.5, 0.9, 3.8, and 7.0 per cent. None of these differences were statistically significant. These data indicate that the incidence of morbidity and mortality that is associated with simultaneous bilateral total knee arthroplasty is no greater than when the procedure is performed during the same hospitalization or separate hospitalizations.