The purpose of this bench-top study was to determine the subatmospheric tracheal pressures that could be generated by different sizes of suction catheters when placed into the different sizes of tracheal tubes encountered in adult anaesthetic practice. Five sizes of suction catheter were studied from 8 to 16 Fr and five sizes of tracheal tubes from 5 to 9 mm internal diameter. We found that the larger suction catheters generated potentially dangerous levels of subatmospheric pressure. If the size of suction catheter was limited so that the ratio of the outside diameter of the suction catheter to the inside diameter of the tracheal tubes was 0.5 or less, tracheal pressures were not more than 2 mmHg subatmospheric. We were able to recommend sizes of suction catheter to use for each size of tracheal tube.