Acute, severe aortic regurgitation due to dilatation of the aortic root was studied in a 16-year-old Japanese female with Takayasu's arteritis. The patient was admitted because of acute pulmonary edema followed by systemic illness characterized by fever, anorexia, and general fatigue. The echocardiogram and aortogram demonstrated acute, severe aortic regurgitation due to dilation of the aortic root. She was successfully treated with aortic valve replacement and steroid. Microscopic examination of the aortic wall demonstrated granulomatous lesions with multinucleated giant cells. Now, three years later, she remains asymptomatic and hemodynamically stable.