Delayed onset of obstruction in the Oxford endotracheal tube during anaesthesia is described in five patients. The effects of intra-cuff voluem and pressure on the wall of the tube were investigated and discussed with special refernce to the ffects of body heat and repeated use on the consistency of the tube. It is concluded that inward collapse of the tube wall is caused by a combination of factors, namely: frequent use, softening of the tube wall by body heat, the gradual increasing of intra-cuff volume and pressure by diffusion of nitrous oxide into the cuff, replacing a damaged cuff by a new one and heat sterilization. Deflation and re-inflation of the cuff to minimal occlusive volume at hourly intervals is suggested as a precautionary measure in the prevention of inward collapse of the tube wall.