A histologic, immunohistochemical, and DNA ploidy analyses were performed on two cases of angiomatoid malignant fibrous histiocytoma to ascertain the histogenesis and relationship of endothelial, histiocytic, and fibroblastic elements. Both cases were slowly growing, grossly encapsulated. Subcutaneous masses resected from pediatric patients. Microscopically, the tumors were composed of solid masses of epithelioid and spindle cells with abnormal endothelial-lined and blood-filled cystic spaces surrounded by normal vascular structures and aggregates of lymphocytes occasionally forming germinal follicles. The tumor cells stained exclusively with CD34 and vimentin antibodies. Tumor-associated vessels stained for CD31, CD34, vimentin, and Ulex europaeus. Occasional cells within germinal follicles stained for lysozyme, CD68, and HAM56. Ploidy analysis of tumor cells showed intermediate aneuploidy with a DNA index of 1.14. Blood vessels within and surrounding the tumor as well as inflammatory cells were DNA euploid. These studies suggest that the tumor--though comprised of histologically and immunohistochemically benign-appearing euploid endothelial, fibroblastic, and inflammatory elements--contains an aneuploid population of undifferentiated mesenchymal cells.