The proposal is made that a number of previously described entities of skin, soft tissue, large vessels, bone, and heart actually constitute different manifestations of the same basic process, characterized by the proliferation of a highly distinctive type of cell descriptively identified as a "histiocytoid endothelial cell." The entities in question are angiolymphoid hyperplasia with eosinophilia and related cutaneous and subcutaneous disorders, atypical vascular proliferation of large vessels, hemangioendothelioma of bone, and endocardial benign angioreticuloma of the heart. The main cell that proliferates in all these conditions has the basic features of an endothelial cell, but also exhibits histochemical and ultrastructural characteristics that are more akin to those of a histiocyte. These unusual features could be the expression of a morphologic abnormality or represent an overgrowth of a specific and as yet undefined subpopulation of endothelial cells, such as Majno's "contractile endothelial cell." Whether this group of proliferative diseases is of a reactive or a neoplastic nature is not immediately apparent, although the latter seems more likely. However, it is clear that the behavior of these lesions, as a group, is quite indolent and even self-limited, in contrast to the aggressive behavior and often fatal outcome of the true angiosarcomas that they so closely resemble on microscopic grounds. The term "histiocytoid hemangioma" is suggested for this group of disorders.