A two-month-old infant developed a vascular tumor of the right flank which upon biopsy proved to be a spindle cell hemangioendothelioma. The increased capillary bed characterizing the neoplasm caused a severe thrombocytopenia together with a consumption coagulopathy (Kasabach-Merritt syndrome). The patient, who was dependent on platelet transfusions, improved quickly after interferon alpha-2a was given at the dosage of 3,000,000 U/m2, with resolution of the Kasabach-Merritt syndrome after three weeks and a 75% decrease of the tumor volume within three months of treatment.