In a prospective, randomized trial, the effectiveness of sterile anesthesia breathing circuits having bacterial filters in decreasing the incidence of postoperative pulmonary infection was evaluated. Two hundred ninety-three patients undergoing major surgical procedures were assigned randomly to receive anesthetics through a sterile, disposable circuit having a bacterial filter, or a reusable circuit that had been washed and dried after previous use. The two groups were well matched for age, sex, type of surgical procedure, type of anesthetic, and history of smoking or obstructive lung disease. A physician observer, who was unaware of these assignments, reviewed the patients' post-operative status for evidence of pulmonary infection. The over-all rate of infection was 3.5 per cent; no significant difference in the rates of infection occurred between the two groups. Five (3.6 per cent) of the 138 patients anesthetized with sterile circuits had postoperative respiratory infection, compared with four (2.6 per cent) of the 155 patients anesthetized with unsterile circuits. In both groups, maximum postoperative temperature, white-blood-cell count, and the lowest values for arterial oxygen tension were similar. The authors therefore conclude that when simple hygienic measures are followed, the routine use of sterile anesthesia breathing circuits with bacterial filters does not decrease the risk of postoperative infection.