An intravascular bronchioloalveolar tumor of lung (IVBAT) was studied with electron microscopy. Based on ultrastructural evidence and information obtained from the literature, we propose the following: (1) IVBAT is a true pulmonary neoplasm with distinctive morphologic features; (2) it consists of cells with endothelial characteristics and is probably derived from multipotential mesenchymal reserve cells; (3) it is not related to the typical bronchioloalveolar tumor of lung; (4) a more appropriate designation for this unusual pulmonary neoplasm is "sclerosing angiogenic tumor."