A patient with spindle cell hemangioendotheliomas was followed from 1964 to the present time, allowing the authors the opportunity to examine the lesions in the early, mature, and old phases. Organizing thrombi of different stages associated with slit-like vascular proliferation were always observed, whereas cavernous vascular spaces predominated as the lesions became older. Each spindle cell hemangioendothelioma initially developed relatively rapidly and was sometimes painful but then persisted as a silent nodule for decades. Transmission and scanning electron microscopic studies revealed that endothelial cells tended to digitate into the slit-like proliferating channels, became attached to other cells by means of tight junctions, and thus obstructed the channels at sites where thrombi developed repeatedly. The vascular spaces, ranging in nature from slit-like to cavernous, were outlined further by a relatively sparse mantle of ramified or dendritic interstitial cells that corresponded to spindle cells. Most of the cells appeared simply to be fibroblasts, but they developed the features of pericytes when they were close to the endothelial lining of well-developed vascular lumens. Large vascular spaces and phleboliths were surrounded by smooth muscle cells. Approximately 20% of the interstitial cells were dendritic macrophages characterized by phagocytic activity, presence of many lysosomes, and Factor XIIIa expression. The long and characteristic clinical course, the histologic evidence that thrombosis and its organization was continually occurring within the lesions, and the ultrastructural finding that spindle cell hemangioendotheliomas were composed of different microvascular segments from capillaries to veins, suggest that spindle cell hemangioendotheliomas may develop from a cycle of recanalization after thrombosis that occurs repeatedly because of the unique endothelial growth that was noted. This is in contrast with the previous conception that they were low-grade angiosarcomas.