Thirty-two cases of so-called sclerosing hemangioma of the lung observed by light microscopy were further studied by electron microscopy and/or immunohistochemistry. Three histologic patterns were seen: hemangioma-like, papillary, and solid. The only significant component representing the nature of the lesion is characteristic round cells within the stroma in all these patterns, whereas the surface cells lining the papillary projections or cystic spaces are normal or are hyperplastic bronchioloalveolar cells with a few neuroendocrine cells. Immunohistochemical findings showed that the "stromal cells" (tumor cells) were positive for neuroendocrine markers, namely, chromogranin A (19 of 22 cases), neuron-specific enolase (24 of 24), synaptophysin (six of 10), adrenocorticotropic hormone (14 of 15), growth hormone (14 of 15), calcitonin (11 of 15), and gastrin (11 of 14). Besides, some tumor cells were positive for epithelial membrane antigen (four of four), carcinoembryonic antigen (one of four), and vimentin (one of one). All tumor cells were negative for polyclonal antikeratin antibody (25 cases), AE1 (one case), and AE3 (one case). However, in contrast to the "stromal cells," the surface cells of the cystic spaces stained positively for keratin (25 of 25 cases), AE1 (one of one), AE3 (one of one), epithelial membrance antigen (four of four), and carcinoembryonic antigen (four of four); only a few of them expressed neruoendocrine markers. Both surface and tumor cells were negative for factor VIII-related antigen (25 cases), CD31 (one case), and α1-antitrypsin (25 cases). Ten cases further studied by electron microscopy and six examined by ultrastructural morphometry showed that the surface cells were mainly type 2 pneumocytes containing many lamellar bodies in the cytoplasm. Lying among them, neuroendocrine cells were occasionally seen. The stromal tumor cells had no lamellar body, but dense core granules (neurosecretory granules) and microtubules. In six cases, 92.3% (345 of 374) of tumor cells contained neurosecretory granules, which were pleomorphic and 73 to 1056 nm in diameter (mean, 302 nm). Two to 193 (mean, 12) neurosecretory granules were found in each tumor cell. Both immunohistochemical findings and ultrastructural evidence indicate that so-called sclerosing hemangioma of the lung is a benign lesion composed of neoplastic neuroendocrine cells with areas of sclerosis. A suggested name for this tumor is benign neuroendocrine tumor of the lung. The differentiation between this tumor and papillary adenoma, bronchioloalveolar carcinoma, or carcinoid tumor of the lung is discussed.