We report the clinical and pathological features of nine distinctive, but relatively unknown, vascular tumors of infancy and childhood presenting as soft tissue masses often associated with locally aggressive disease, lymphangiomatosis and Kasabach-Merritt syndrome. The patients, four males and five females, were all in their first decade of life except for two (median, 2 years; range, 5 months to 19 years). These tumors involved deep soft tissues of the upper extremity (four cases), retroperitoneum (two cases), chest wall, scalp, and neck (one case each). Four patients also had Kasabach-Merritt syndrome, and three patients had lymphangiomatosis. Lymphangiomatosis consisted of diffusely infiltrating lymphangioma of soft tissue (three cases) and in two by the additional presence of bone lesions. In one of these three cases, lymphangiomatosis antedated the diagnosis of the vascular tumor, and in the remainder they were concurrently diagnosed. Tumors were characterized by infiltrating, interconnecting sheets or irregular nodules of slender endothelial cells lining crescentic or slit-like vessels and, less commonly, rounded capillary-type vessels. Within some tumors, nests of epithelioid endothelial cells with prominent eosinophilic cytoplasm containing finely granular hemosiderin, hyaline droplets, and cytoplasmic vacuoles were identified. Smaller amounts of hemosiderin were observed within the spindled endothelial cells and microthrombi could be seen occasionally within the tiny lumina. Nuclear atypia was minimal within these tumors and mitotic figures were infrequent, averaging 2 to 3/10 high-power fields (HPF) (range 0-7/10 HPF). Larger, well-formed feeding vessels were present at the periphery of the tumor. The endothelium of these vessels expressed factor VIII-AG, CD34, and bound Ulex europaeus, and contained an occasional perithelial cell expressing muscle-specific actin. In contrast, the spindled tumor cells expressed only CD34. Human papilloma virus (HPV)-16-like DNA transcripts, which have been identified in cases of Kaposi's sarcoma, were not detected by polymerase chain reaction in two cases. Follow-up information revealed that four patients were alive without disease after wide excision, multiple excision(s), or amputation (one case); three were alive with disease; and two died, one from lymphangiomatosis with respiratory compromise and the other from hemorrhage complicating Kasabach-Merritt syndrome. It appears that treatment should consist of wide local excision and supportive therapy for associated symptoms.