We report a 43-year-old man who presented diabetic ketoacidosis 1 year after receiving kidney transplantation. He was a recipient of renal transplantation treated with metyl-prednisolone and tacrolimus regimen. The serum level of tacrolimus was 12.4 ng/ml, and he showed hyperphagia before a month of admission. A week before admission, he was aware of polydipsia, polyuria, and general fatigue. He visited our hospital and was found to have severe hyperglycemia (925 mg/dl), significant ketosis and mild metabolic acidosis (pH 7.341), although he had not been diagnosed as diabetes mellitus. He administrated in our hospital, and was treated with insulin for 5 weeks. He was not obese (BMI = 18.2 kg/m(2)) and had no family history of type 2 diabetes. He was finally treated with diet therapy alone. The 24 h urine C-peptide secretion on the third hospital day was low (8.4 microg per day). However, no autoantibodies against pancreatic islets were positive, and his insulin secretion was recovered at discharge suggesting that he was not type 1 diabetes. Although, tacrolimus has been reported to cause or worsen diabetes mellitus, the present case suggests that it could cause severe decrease in insulin secretion which leading to diabetic ketoacidosis in lean subject without previous history of diabetes mellitus.