A case of large ameloblastoma of the mandible with severe hypoproteinemia is reported. The patient, a 53-year-old man, had a large swelling (14 x 11 x 10 cm) from the right cheek to the submandible causing severe hypoproteinemia. The tumor was removed when the serum protein level had recovered to 5.3g/dl following administration of 50 ml of 20% albumin for 4 days. After the operation, his hypoproteinemia, which was thought to be caused by leakage of plasma through the oral fistula of the ameloblastoma, rapidly improved. This article describes preoperative and postoperative findings and the clinical course of the patient.