We present a case of a congenital angiomatoid malignant fibrous histiocytoma. This rapidly growing lesion, which was located in the subcutis of the left upper arm, was excised at the age of 8 1/2 months. The patient, a girl, was well and free of disease 10 months after surgical removal of the tumor. The tumor appeared grossly encapsulated. The gray-tan tissue contained cystic spaces filled with recent and organizing hemorrhages. Microscopically, the tumor was composed of solid masses of histiocyte- and fibroblast-like cells, inflammatory infiltrate, and multifocal irregular blood-filled spaces, which were predominantly devoid of endothelial cells. The tumor was studied immunohistochemically with antibodies specific for FVIII-related antigen, S-100 protein, epithelial membrane antigen, vimentin, desmin, alpha-1-antitrypsin, muramidase, laminin, and collagen type IV. Ulex europaeus lectin-I was also utilized. These studies, along with our ultrastructural findings, suggest that: (a) the tumor is composed of a mixture of mesenchymal cells; (b) an imperfect angiogenesis may be taking place, resulting in a wide spectrum of vascular structures; and (c) the cell of origin may be a pluripotent mesenchymal cell.