A heterogeneous group of five patients with progressive, invasive angiomatous diseases including pulmonary hemangiomatosis, angiosarcoma, or massive hemangioma with associated consumptive coagulopathy were treated with interferon alfa-2a for periods of 17 to 33 months. One patient with a large thoracic hemangioma, cardiorespiratory failure, and consumptive coagulopathy died after less than 2 months of treatment. The remaining four patients have shown beneficial responses, including (1) regression of abnormal vessels on pulmonary angiogram and improved exercise tolerance in pulmonary hemangiomatosis (two patients), (2) decreased corticosteroid and/or platelet transfusion requirements in consumptive coagulopathy (two patients), and (3) decreased size and number of tumor nodules in the one patient with angiosarcoma arising in preexisting angiomatous lesions. Responses occurred during periods of 2 to 20 months of treatment. There was no measurable progression of angiomatous lesions in any patient receiving interferon at the therapeutic dose, except possibly in the one who died. Each of the four surviving patients had improved linear growth and weight gain during interferon treatment.