We evaluated the clinical and radiologic results and the immobilizing effect of the Knight-Kim brace for bony union in 75 consecutive cases of spondylolisthesis treated by anterior interbody fusion. The fifth lumbar vertebra was the most common site involved (55%). The patients were followed for a minimum of 2 1/2 years (range: 2 1/2 to 14; average: 4.5). All cases were approached retroperitoneally, and autogenous bone graft was obtained from the iliac crest. The donor site was reconstructed by polymethylmethacrylate bone cement. Among the 75 cases of grade I, II, and III slippage according to Meyerding's grading system, 58 cases (77%) showed complete bony union after 1 year follow up. Excellent and good clinical results were obtained in 65 cases (87%) at 1 year postoperatively. The state of bony union did not always correlate with the clinical symptoms. We conclude that the simple Knight-Kim back brace was valuable for postoperative immobilization.