Scanning electron microscopy and transmission electron microscopy were applied to lung biopsy specimens from patients with congenital heart defects, and pulmonary artery endothelium was analyzed for alterations in surface characteristics and intracytoplasmic composition which might reflect abnormal function. The patients were divided into four groups distinguished by increasing severity of pulmonary vascular changes on light microscopy graded both morphometrically and by the Heath-Edwards classification; group 1, normal vasculature or only abnormal extension of muscle into peripheral arteries; group 2, medial hypertrophy; group 3, medial hypertrophy +/- decreased artery number + intimal hyperplasia; group 4, decreased artery number + occlusive intimal hyperplasia. On scanning electron microscopy, the pulmonary artery endothelial surface in group 1 patients was "crinkled" or "corduroy-like", i.e., composed of narrow, even ridges; in groups 2 and 3, it was "cable-like", i.e., comprised of deep intertwined ridges; in group 4 it was "chenille" in texture, i.e., high ridges alternated with low, uneven, and twisted ones. There was significant increased density of surface microvilli in groups 2 and 3 patients when compared to groups 1 and 4 (p less than 0.05 for each comparison). On transmission electron microscopy pulmonary artery endothelial cells in groups 2 and 3 patients were also characterized by a significant increase in the volume density of rough endoplasmic reticulum (p less than 0.01) and microfilament bundles (p less than 0.05). The coarse endothelial surface characteristics associated with pulmonary vascular changes may result in abnormal interaction with blood elements and release of vasoactive substances. The increased microvilli, rough endoplasmic reticulum, and microfilament bundles in patients with moderate but not advanced arterial changes suggest a phase where increased endothelial metabolic function and alterations in the cytoskeleton may also contribute to heightened pulmonary vascular reactivity.