The epithelium of 1549 sections of the tracheo-bronchial tree of 35 children showed no atypical cells in any of the sections nor a single lesion resembling a carcinoma-in-situ. Epithelial changes were found in 16.6% of the 1549 sections. This compares with 16.8% of sections of adults who never smoked regularly. The epithelial changes in children were those chiefly of squamous metaplasia and stratification (14.6%) and only 2.4% were those of basal cell hyperplasia. In adults, 6.0% of the sections showed squamous metaplasia and stratification and 9.4% showed basal cell hyperplasia. The reasons for this difference may lie in the fact that the majority of lesions in children appear to be the result of the repair of ulcers. In this series of children studied there appears to be no relationship between ulceration and pneumonia at the time of death.