Lung sections from 22 infants and children who died of noncardiopulmonary diseases (control) and from 12 infants with hyaline membrane disease/bronchopulmonary dysplasia (HMD/BPD) were immunostained for bombesin (BOM), calcitonin (CT), calcitonin gene-related peptide (CGRP), leu-enkephalin (ENK), and serotonin (5HT). The numbers of immunoreactive cells per millimeter airway epithelial length were counted and determined by morphometry. In lungs from the control group, BOM-immunoreactive (IR) solitary cells (SCs) were numerous at the bronchiolar level, and remained so for 4 to 7 months after birth. The number of CT-IR SCs increased markedly around term. At the bronchial level these cells continued to increase postnatally apparently in inverse proportion to the number of BOM-IR SCs in later childhood. The number of CGRP-IR SCs was high only during the neonatal period. 5HT-IR SCs were relatively few and showed no clear developmental change in number. No unequivocal ENK-IR SCs were observed in any case. BOM-IR neuroepithelial bodies (NEBs) were observed more frequently than any other type of NEBs and remained relatively numerous throughout childhood. In lungs of infants with HMD/BPD, endocrine cells of all types except for ENK-IR cells were markedly increased in number during periods of regeneration, to the chronic stage. It is concluded that each of these types of pulmonary endocrine cell demonstrates a characteristic developmental pattern in distribution and frequency during infancy and childhood, probably reflecting the various functional roles of these cells in early life. In HMD/BPD, there is alteration in the number and/or peptide and serotonin content of pulmonary endocrine cells, possibly a result of acute hypoxia, oxygen administration, and epithelial regenerative activity.