In order to evaluate whether the risk of bacterial contamination increased during low-flow anaesthesia without bacterial filter compared with non-rebreathing anaesthesia with a disposable bacterial filter, two groups of patient were studied. In the first group a disposable circle absorber system was changed once daily, thus it was used for several patients. In the second group a non-rebreathing system was connected to a disposable bacterial filter before each surgical procedure. Samples for microbiological examination were taken preoperatively from the oropharynx and postoperatively from three locations in the circle system and in the non-rebreathing system. No difference in rates of circuit contamination were observed between the two groups.