A prospective study of factors that might contribute to the development of acquired subglottic stenosis was undertaken in newborn infants with endotracheal tubes in place for 7 days or more. Duration of intubation, the number of endotracheal tubes inserted, the duration of mechanical ventilation, the presence of post-extubation stridor, and the size of the endotracheal tube in relation to gestational age significantly correlated with the development of subglottic stenosis. Patients at risk for significant subglottic stenosis were those with post-extubation stridor and those with tubes in place for 25 days or longer. On the basis of these findings, it is recommended that endotracheal tubes be chosen such that the ratio of nominal tube size divided by the patient's gestational age in weeks is less than 0.1.