The treatment of massive osteolysis with lymphangioma and/or hemangioma (Gorham-Stout syndrome) has been controversial. The authors report on a patient with multiple massive osteolyses and extensive lymph-hemangiomatosis whose lesions were reduced by interferon alfa therapy. A 2-year-old girl had complained of left chylothorax. Thoracoscopy showed an increase in small lymphatic vessels in the chest wall. The chylothorax was improved by coagulation of the lymphatic vessels. Later, multiple massive osteolyses appeared in the left 11th and 12th ribs, the TH10-L3 vertebrae, and the right femur. There were also hemangiomas in the liver and spleen, a tumor lesion in the left lower chest wall, and hemangiomatous change on the skin surface of the left back. The left lung had only a minimal air content. After OK-432 was injected into the femur and chest wall lesions, the femur lesion disappeared. Then, as right chylothorax appeared, OK-432 was injected into the right pulmonary cavity. The chylothorax disappeared, but pericardial effusion appeared. After steroid pulse therapy, pericardial effusion disappeared. During these treatments, the 7th to 10th ribs disappeared from the x-ray and scoliosis developed. One month later, a cloudy fluid collection in the right lung was found on computed tomography. Interferon alfa and steroid pulse therapy were started. Interferon alfa (1,500,000 units) was subcutaneously administered daily for 2 months and was gradually reduced and maintained at 1,500,000 unit/wk. Steroids were also reduced and maintained at 5 mg/d of predonine. Later, the progress of osteolysis and the extension of lymph-hemangiomatosis stopped. Ten months later, hemangioma in the back disappeared, and the 7th to 10th ribs, which had disappeared, reappeared. The interferon alfa therapy was stopped 14 months after it was administered. The patient's condition has been stable for 10 months since then. At this time, computed tomography shows regression of the hemangiomatous lesion in the back. The authors clinically diagnosed the patient as having Gorham-Stout syndrome with extension of lymph-hemangiomatosis. Interferon alfa with or without steroid therapy should be a choice for patients with extension lesions.