A 62-year-old man with diabetes presented with general malaise, shortness of breath, and bradycardia. His medical history included retroperitoneal fibrosis, enlargement of the pancreas, and symmetrical swelling of the lacrimal glands with elevated serum IgG4 levels (175 mg/dL). He had received steroid therapy for 1 year for suspected IgG4-RD. A physical examination revealed a blood pressure of 90/42 mmHg, a pulse rate of 42 b.p.m., and body temperature of 36.6°C. An early diastolic murmur was noted on auscultation. The laboratory test results revealed elevated levels of white blood cells (WBC, 17 800/mm3), C-reactive protein (CRP, 4.07 mg/dL), and erythrocyte sedimentation rate (ESR, >120 mm) in the first hour. The serum IgG4 level was within the normal range (101 mg/dL) under oral prednisolone treatment (10 mg/day). An electrocardiogram revealed a complete atrioventricular block. A chest radiograph showed a normal cardiac silhouette and clear lung fields. Although echocardiography revealed a normal aortic valve 1 year ago, a transthoracic and transoesophageal echocardiography now revealed a thickened tricuspid aortic valve and the LVOT wall with severe aortic regurgitation, but no evidence of aortic stenosis and LVOT obstruction. The left ventricular (LV) systolic function was preserved, without wall motion abnormalities, and the LV end-diastolic diameter was slightly increased. A contrast-enhanced computed tomography (CT) of the chest showed a thickened aortic valve extending to the LVOT wall and normal thickness of the ascending aortic wall. A cardiovascular magnetic resonance (CMR) revealed a high-intensity signal around the aortic valve in the late gadolinium enhancement, whereas there was no significant change in the myocardium, the ascending aortic wall, and the surrounding structures. We increased the oral prednisolone to 30 mg/day after three days (1 g/day) of high-dose methylprednisolone for clinically suspected IgG4-RD of the aortic valve. The complete atrioventricular block improved to a first-degree atrioventricular block within a few days. Thus, he avoided permanent pacemaker implantation. He received an angiotensin receptor blocker (olmesartan 10 mg/day) and a long-acting loop diuretic (azosemide 60 mg/day) as a medical therapy for aortic regurgitation-related heart failure. The inflammatory markers CRP and ESR gradually returned to within the normal range, and his serum IgG4 level decreased further (56.3 mg/dL). A follow-up echocardiography demonstrated a slight regression of the thickened aortic valve and the LVOT wall. We thus decreased the oral prednisolone to 25 mg/day after 1 month of administration and initiated azathioprine (50 mg/day). A follow-up contrast-enhanced CT showed regression of the thickened aortic valve and the LVOT wall. Nevertheless, he presented with worsening shortness of breath and malaise three months after increasing the corticosteroid dose. Plasma brain natriuretic peptide increased from 69 pg/mL to 414 pg/mL, whereas the serum IgG4 level decreased to 27.6 mg/dL. The chest radiograph showed pulmonary congestion and cardiomegaly. The transthoracic echocardiography revealed the progression of aortic regurgitation and an enlarged LV dimension, whereas the aortic valve and LVOT wall thickening regressed. The LV systolic function was preserved, with no evidence of significant aortic stenosis and no LV wall motion abnormalities. He underwent aortic valve replacement for severe symptomatic aortic regurgitation and heart failure deterioration. The patient received a bioprosthetic aortic valve (25-mm Carpentier-Edwards PERIMOUNT; Edwards Lifesciences, Irvine, CA, USA), as we were concerned about the risk of anticoagulation-related bleeding and the possibility that there could be problems further down the line with complications from multiple organs and ongoing anticoagulation. During the operation, we observed the thickening and shortening of the tricuspid aortic valve. The entire LVOT wall was also thickened. In contrast, the ascending aorta was normal. The pathological examination of the excised valve leaflets showed a dense lymphoplasmacytic infiltrate mixed with fibrotic tissue. The immunohistochemical staining revealed a ratio of IgG4-positive plasma cells to IgG-positive plasma cells greater than 0.5. Thus, he was diagnosed with IgG4-RD of the aortic valve. The dose of prednisolone was tapered by 2.5 mg every 2 months in combination with azathioprine 50 mg after surgery. The transthoracic echocardiography 1 year after the surgery showed regression of the LV dilatation and normal function of the prosthetic valve.