We describe two children with vascular neoplasms that resembled Kaposi's sarcoma in places. Both presented with intraabdominal masses and severe thrombocytopenia. At autopsy the tumors extensively infiltrated the peritoneum and retroperitoneum and surrounded or invaded numerous organs including the kidneys, pancreas, adrenal glands, gastrointestinal tract, mesentery, and lymph nodes in both cases, and spleen or bone marrow in one case each. The neoplasms were histologically identical and displayed two patterns: dilated vascular spaces (angiomatous areas) lined by flat endothelial-like cells and areas of spindle cells forming slitlike vascular spaces similar to those described in Kaposi's sarcoma. Tumor cells in both cases expressed markers for endothelial cells. The clinical and histologic character of these neoplasms differentiates them from Kaposi's sarcoma, hemangioendothelioma, and from conventional juvenile hemangioma.