The development of the tracheal submucosal glands has been determined quantitatively in 22 infants with cystic fibrosis and in 25 control infants, all under 4 months of age. In cross-sections of normal trachea significant relationships were found between postconceptional age (PCA) and gland area (P less than 0.001), submucosal area (P less than 0.02), tracheal airway diameter (P less than 0.05), and acinar diameter (P less than 0.001). In infants with cystic fibrosis the pattern of development was similar to that of the control infants. No statistically significant differences were found between three subgroups of infants with cystic fibrosis, which included those with meconium ileus with no lung infection, those with meconium ileus with lung infection, and those with lung infection and no history of meconium ileus. The normal pattern of development of tracheal submucosal glands in infants with cystic fibrosis was in contrast to the deficiency of normal maturation seen in the exocrine pancreas of these infants. The lumen fraction, an index of dilatation of acinar lumina, showed no significant relationship with PCA in either the control group or the group with cystic fibrosis. However, statistically significant dilatation of acini was observed in the tracheal submucosal glands of infants with cystic fibrosis (0.14, P less than 0.005).