The contribution of an abnormal pulmonary vascular bed to right-to-left shunting in patients with congenital left-sided diaphragmatic hernia and alveolar hypoplasia has not been defined. In three infants we analyzed lungs fixed by perfusion. Left lung volumes (ml/kg birth weight) were 1.7, 6.5 and 4.0 respectively (control = 11.7) and right lung volumes 5.7, 11.7 and 9.8 respectively (control = 14.3). Serial sections were prepared and fifth generation (resistance) vessels identified. Compared to control the medial widths were increased in all lungs and the medial width/external diameter ratios were increased in the left lungs of two patients and the right lung of one. The m/d ratio in the left lung was greater than that in the right lung in two of the three patients. There were fewer pulmonary vessels/cm2 lung tissue in the study lungs than in the control lung, and there were significantly fewer pulmonary vessels/cm2 lung tissue in the left than in the right lung in two of the three patients. Although there was increased smooth muscle in resistance vessels, intravenous therapy with tolazoline HCl did not improve systemic oxygenation. We conclude that this was probably due to the decreased total size of the pulmonary vascular bed and the decreased number of pulmonary vessels per unit lung tissue, causing a fixed high pulmonary vascular resistance.