The effect on endotracheal tube flammability of helium (He)-diluted oxygen and He-diluted nitrous oxide was determined with the oxidant O2(He) and oxidant N2O (He) indices of flammability. These values then were compared with the corresponding values for nitrogen dilution,i.e., oxidant O2(N2) and oxidant N2O (N2) indices of flammability. Four different types of endotracheal tubes were studied: polyvinylchloride (PVC), red rubber (RR), silicone (Si), and the Xomed (Xo) Lasershield® tube. The oxidant O2(He) indices are: PVC 0.274 ± 0.0055, RR 0.194 ± 0.0089, Si 0.194 ± 0.0055, and Xo 0.256 ± 0.0055. The oxidant N2O (He) indices are: PVC 0.526 ± 0.0055, RR 0.434 ± 0.0114, Si 0.416 ± 0.0055, and Xo 0.456 ± 0.0154. The oxidant O2(N2) indices are: PVC 0.254 ± 0.0055, RR 0.182 ± 0.0045, Si 0.200 ± 0, and Xo 0.230 ± 0. The oxidant N2O (N2) indices are: PVC 0.472 ± 0.0084, RR 0.356 ± 0.0055, Si 0.392 ± 0.0045, and Xo 0.444 ± 0.0114. These differences, although statistically significant, are modest and demonstrate only a small, and probably clinically not significant, preference of He over N2in decreasing flammability.