Though acute rheumatic fever (RF) is now rare in Japan, it continues to be an important disease condition that physicians should be prepared to diagnose and treat. We describe a patient with acute RF accompanied by transient aortic regurgitation (AR). The AR was detected only by echocardiography. There were no other indications, and it disappeared after treatment with prednisolone. The changes in cardiac valves in the early phase of RF have been the subject of only a few case studies. Echocardiography is quite valuable in the workup of patients with acute RF and should be performed even if there are no signs of cardiac involvement.