The authors have previously shown that fetal tracheal ligation (TL) reverses the pulmonary hypoplasia in experimental diaphragmatic hernia (DH) by accelerating fetal alveolar growth. The purpose of this study was to determine if growth of the accompanying macroscopic and microscopic pulmonary vasculature is also accelerated. Eighteen fetal lambs were divided into three experimental groups: diaphragmatic hernia (DH), DH and simultaneous tracheal ligation (DH/TL), and sham-operated controls (C). Animals were delivered near term, the lungs retrieved, and pulmonary capillary growth (5 to 50 microns in diameter) evaluated by standard morphometric techniques. Capillary ultrastructure was evaluated by electron microscopy. Nine additional fetal lambs of the same gestational age were equally divided into the same three groups and their lungs analyzed by pulmonary arteriography for evaluation of large vessel growth (< 100-microns diameter). Computer digital analysis of angiogram lung slices showed that the total area of large vessels was increased in DH/TL lungs when compared with DH lungs and decreased in DH lungs when compared with C lungs (P = .003); however, the ratio of large vessel area per unit of lung area was similar in all groups. Microscopic morphometry of the capillary bed showed that the total number of capillaries was increased in DH/TL lungs over both DH and C lungs (P = .0001); however, the number of capillaries per alveolus (cap/alv) was similar in all groups. In DH/TL lungs, electron microscopy showed normal capillary wall structure and normal thickness of the capillary-alveolar interface, whereas in DH lungs, capillary structure was abnormal and the capillary-alveolar interface was thickened.(ABSTRACT TRUNCATED AT 250 WORDS)