Endotracheal tube (ETT) fire has been reported secondary to laser and electrocautery ignition. The flammability of polyvinylchloride (PVC), silicone (Si), and red rubber (RR) ETTs in oxygen (O2) and/or nitrous oxide (N2O) in nitrogen was determined and compared by means of the O2 and N2O indices of flammability. The O2 index of flammability is the minimum O2 fraction in nitrogen that will support candle-like flame using a standard ignition source. The O2 index of flammability for PVC ETTs is 0.263, for Si 0.189, and for RR 0.176. The N2O index of flammability is the minimum N2O fraction in N2 that will support candle-like flame using a standard ignition source. The N2O index of flammability for PVC ETTs is 0.456, for Si ETTs 0.414, and for RR ETTs 0.374. The indices are additive. Flammability is a valid method of comparing safety of various endotracheal tube materials. There is a need for new endotracheal tube material with a higher index of flammability. The significance of these findings and the clinical applications are discussed.