The substitution of guanine for adenine at position 3243 of the leucine tRNA gene of mitochondrial DNA was originally described in association with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes). Diabetes mellitus associated with the mutation (mitochondrial diabetes) is a different phenotype from MELAS. We identified 11 patients with the mutation among 385 Japanese diabetic patients: two had MELAS and nine had mitochondrial diabetes. We present data on a male patient with mitochondrial diabetes who developed the nephrotic syndrome at the age of 23. Light microscopy revealed mesangial expansion, PAS-positive deposits and segmental sclerosis in the glomeruli. Scattered mesangial electron-dense deposits and thickening of the basement membrane were found on electron microscopy, suggesting that diabetic glomerulosclerosis accompanied by focal glomerulosclerosis (FGS). Mitochondrial diabetes may pre-dispose patients to renal complications, including forms of glomerulonephritis, such as FGS.