Tracheal suction catheters damage the mucosa of the airway, due to both the pull from the vacuum and insertion of the catheter. The pull of the vacuum from a suction catheter was measured on a Mettler balance. An open side hole reduced the pull by around 90% and a distance of 1 mm gave a reduction of around 90%. The combination of one side hole and a 1 mm distance gave a 99% reduction of the pull. The difference between one or more side holes was measured. The suction efficacy was higher with a single side hole than with several side holes. On the basis of these measurements and a review of the literature, the principles were outlined for the construction of a suction catheter which provided maximum suction efficacy and caused minimum damage: (1) a moderate to strong vacuum was to be used; (2) the catheter was to have a single hole at the tip and a single side hole placed at the convex side of the angle; (3) both side hole and tip hole should be so constructed that the holes were kept at a distance from the mucosa; and (4) the tip should be angulated.