Significant aspiration may occur around correctly inflated high volume, low pressure endotracheal tube cuffs. The prevention of silent aspiration due to leaks around cuffs of endotracheal tubes was investigated during general anesthesia for hip replacement in 47 patients. The patients were randomly assigned to one of three groups, in which one of three endotracheal tubes of different designs were used for intubation. The following three tubes were used: the red rubber Rüsch tube with low residual volume cuff, inflated to minimum occluding volume; the Mallinckrodt Hi-Lo tube with high residual volume cuff; the NL tube with high residual volume cuff and automatic cuff pressure regulation. The cuffs on the Mallinckrodt and the NL tubes were inflated to 29-31 cm H2O. One hour before termination of the surgical procedure, 1 ml methylene blue dye was injected into the trachea just above the cuff through a thin channel built into the tubes. At termination of the operation, the trachea below the cuff was inspected with a fiberoptic bronchoscope. Aspiration was found in 12.5% with the Rüsch tube, in 31.2% with the Mallinckrodt tube, and in 0% with the NL tube. Our results show that silent aspiration is still a problem with standard endotracheal tubes, but that it may be minimized by use of appropriate tubes, cuffs, and control of cuff inflation.