To elucidate the apparent contradictions in vascular remodeling in the lungs of patients with idiopathic pulmonary fibrosis, we evaluated alveolar vascularity in relation to the various degrees of fibrosis in surgically biopsied lungs of usual interstitial pneumonia. Alveolar capillary endothelial cells were intensely immunoreactive with CD34 but not with von Willebrand factor. Vascular density, that is, the relative ratio of capillary area to total area of alveolar walls, was significantly higher at low grades of fibrosis than in control lungs, whereas vascular density gradually decreased as the degree of fibrosis increased and was lower than that of control lungs in the most extensively fibrotic lesions. No vessels were observed inside fibroblastic foci. The potent angiogenic factors vascular endothelial growth factor and interleukin-8 were abundantly produced by capillary endothelial cells and alveolar epithelial cells in highly vascularized alveolar walls. In contrast, venules with CD34-negative but von Willebrand factor-positive endothelial cells localized in the center of the fibrotic lesions were slightly increased and identified as postcapillary venules by three-dimensional reconstructed images. These results indicate the presence of heterogeneous vascular remodeling in usual interstitial pneumonia.