Light microscopic morphometric techniques were used to study the inflated airways of 17 normal infants from 22 wk of gestation to 8 months postnatal age, plus three young adults. In addition, the lungs of four premature and seven mechanically ventilated infants were studied. A computer-assisted digitizer was used to measure airway diameter and to determine the area of cartilage, gland, and muscle in each type of airway. Epithelial height was also measured, and goblet cell number was counted. Using histochemical stains, types of glycoprotein were identified in goblet cells and submucosal glands. Between 22 wk of gestation and 8 months of age, the normal infant showed a linear increase with age in airway diameter. With age the area of muscle and submucosal gland increased in total amount and showed a significant linear increase when related to the size of the airways, except the submucosal gland in hilar airways. By contrast, the increase in cartilage was only commensurate with the increase in size of intrapulmonary airways, but showed a linear size-related increase in the main bronchus. Epithelial height and goblet cell number increased with age in the bronchi, but not more peripherally. Premature infants had for their postconceptional age normal-sized airways with an increase in amount of bronchial smooth muscle and an increase in number of goblet cells. Ventilated infants had a greater increase in smooth muscle (p less than 0.05 to p less than 0.002) and goblet cells (p less than 0.05) and an increase in submucosal gland area (p less than 0.04) whether or not the infant had had hyaline membrane disease.