The incidence and structure of arterial bronchopulmonary anastomoses and pulmobronchial arteries were studied by microangiographic and histologic techniques in material from 37 immature, premature, and full-term neonatal infants and 15 infants and children varying in age from 3 weeks to 4 years, 7 months. The number of arterial bronchopulmonary anastomoses was found to increase with postnatal age. The opposite was true for the pulmobronchial arteries, and some anastomoses were probably formed from pulmobronchial arteries which established precapillary communication with adjacent branches of true bronchial arteries. From the age of 2½ months, many anastomoses are narrowed by intimal smooth muscle cells; and, from the age of 7 months, some anastomoses are completely obliterated by smooth muscle cells and fibrous tissue. The majority of the arterial bronchopulmonary anastomoses probably become obliterated as the individual approaches adulthood.