Kaposiform hemangioendothelioma is an aggressive vascular proliferation that has been recognized as a separate entity from other childhood vascular tumors. The authors present an unusual case of hemangioendothelioma arising in the mediastinum of a 7-month-old male infant with acute respiratory distress and Kasabach-Merritt syndrome. This patient with life-threatening lesion showed a dramatic response to interferon alfa-2a. This case had many histologic and clinical features consistent with kaposiform hemangioendothelioma.