A 40-year-old man was transferred to our hospital because of severe anasarca. He was a heavy drinker for more than 20 years, and diagnosed with diabetes mellitus 8 years earlier and treated with retinal photocoagulation 8 months earlier. He reported loss of appetite after divorce 10 months prior to admission. On admission, he presented with systemic edema and dyspnea. Chest radiography showed massive pleural effusion and cardiomegaly. Serum total protein was 5.6 g/dl, albumin 2.6 g/dl, and urinary protein excretion was 5.3 g/day. Glucose tolerance test showed normal pattern. Ultrafiltration and continuous hemofiltration resulted in loss of 40 kg body weight in 5 days. Echocardiography revealed high-output heart failure and blood tests showed low serum thiamine level of 12 ng/ml (normal, >28 ng/ml). Accordingly, the diagnosis was established as beriberi heart disease complicated with nephrotic syndrome. Treatment with 50 mg/day thiamine intravenously and 80 mg/day furosemide resulted in increase in urine output, decrease in cardiac output, resolution of pulmonary effusion, and about 70 kg body weight loss. Percutaneous renal biopsy showed nodular glomerulosclerosis, mesangial matrix expansion, and thickening of glomerular basement membrane (GBM). Immunofluorescence study showed no glomerular deposition of immunoglobulin or complement. Electron microscopy showed GBM thickening and mesangial matrix deposition without electron-dense deposits or fibrils. These findings were compatible with diabetic glomerulosclerosis. In this patient, extreme malnutrition altered glucose tolerance but, on the other hand, nephrotic syndrome associated with diabetic nephropathy made the diagnosis of beriberi heart disease difficult.