Masson's vegetant hemangioendothelioma is a benign intravascular tumor, sometimes confused with such malignant vascular tumors as angiosarcoma, whose clinical appearance is nonspecific and whose diagnosis can only be established through microscopic examination. The fine needle aspiration (FNA) cytologic findings together with the histologic and immunohistochemical findings of such a tumor in a 22-year-old man are presented. Cytologically, the material obtained by FNA consisted of two distinct populations of cells. One type of cell had large nuclei (sometimes displaced towards the periphery), finely granular chromatin, prominent nucleoli and abundant globular cytoplasm. The second type of cell had spindle-shaped or oval nuclei, granular chromatin and scanty eosinophilic cytoplasm. Although these findings may permit the cytologic recognition of this entity, the aspirate in this case was interpreted as showing a vascular mesenchymal tumor of probable malignancy. Histologic examination of the excised tumor, aided by immunoperoxidase studies for factor VIII antigen that revealed the endothelial nature of the proliferating cells, established the correct diagnosis.