In a private 160-bed orthopedic hospital, the effect of air decontamination and antibiotic prophylaxis on postoperative infections was studied. From 1971 to 1978, 5 016 orthopedic procedures were performed without antibiotic prophylaxis. Postoperative infection rate was 2% with the conventional operating room and 1.9% with the sterile room. The beneficial effect of vertical laminar air flow thus appears doubtful. From 1979 to 1982, prophylactic therapy with a cephalosporin was used for total hip replacements. Rate of postoperative deep infection fell from 3.3% in 811 hip replacements without antibiotics to 0.5% in 1 172 replacements with antibiotic prophylaxis. Our findings suggest that cephalosporin prophylaxis is more effective than vertical laminar air flow for preventing postoperative hospital-acquired infection.