The clinicopathologic features of 50 noninfiltrating angiolipomas are presented, with a detailed ultrastructural study of five cases and immunoflourescence study of one case. The clinical presentation, including age, location, and associated symptoms, is discussed. Pertinent light microscopic findings, including features helpful in diagnosing borderline lesions, are described. The first ultrastructural and immunofluorescence study of this entity is presented. Among various ultrastructural findings, there appear to be decreased numbers of Weibel-Palade bodies in the endothelial cells of all angiolipomas studied. The relationship of Weibel-Palade bodies to the functional state of the cell and their possible procoagulative effect are discussed. Immunofluorescence and ultrastructural studies further suggest that endothelial disruption and endothelial fibrinogen contribute to the fibrin thrombus formation typically found in angiolipomas.