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The morphological features of a spontaneous, multifocal vascular neoplasm of chickens are described. Histologically, the tumor was characterized by areas consisting of freely anastomosing vascular channels with prominent papillary appearance and lined by bland-looking endothelial cells, which alternate with areas resembling cavernous hemangioma. Occasionally solid areas composed of plump, pleomorphic cells were also present. Although there was no clear evidence for metastatic spread, some tumors were obviously invasive. Electron microscopy and immunohistochemistry confirmed the endothelial nature of neoplastic cells, demonstrating in particular pinocytic vesicles, well developed junctional complexes, fragmented basal lamina, occasional Weibel-Palade bodies, and patchy factor VIII-related antigen immunoreactivity. The overall appearance of the tumor was that of a cavernous hemangioma with prominent papillary endothelial hyperplasia. Previously we have shown that the tumor was induced by a newly isolated strain of avian hemangioma retrovirus and in this study we demonstrated typical type C retrovirus particles in the tumor by electron microscopy. It is suggested that this retrovirus-induced avian tumor may serve as a useful model for the study of transformed endothelia and other vascular tumorigenesis.