In a previous study of factors influencing the development of acquired subglottic stenosis (SS) we proposed choosing an endotracheal tube (ETT) size such that the ratio of the external diameter divided by the gestational age of the infant in weeks was less than 0.1. We have prospectively followed 44 patients intubated longer than 25 days. One of 36 patients intubated according to the above protocol developed significant SS compared to 3 of 8 patients intubated with an ETT larger than suggested. In a previous study, 9 of 49 patients intubated longer than 25 days developed SS. The incidence of SS in patients intubated with "appropriate-sized" ETT was less than either other group of patients (P less than 0.005 and P less than 0.05, respectively). This study confirms the suggestion that choosing ETT size by the above protocol will lessen, although by itself not eliminate, the incidence of acquired SS.