Cyproterone acetate is an antiandrogenic drug which has been used in the treatment of advanced prostatic cancer. The drug is thought to be well-tolerated but may impair carbohydrate metabolism. Parameters of carbohydrate metabolism such as fasting blood glucose and hemoglobin A1C should be examined carefully in all patients and particularly in all patients with diabetes before and regularly during therapy with the drug. An 80 year-old man treated with cyproterone acetate 200 mg/day for advanced prostate cancer presented with fatigue. He had type 2 diabetes and was treated with oral antidiabetic medications. His blood glucose level was 555 mg/dL and serum osmolality was 309 mosm/kg. Hyperosmolar hyperglycemic state (HHS) was diagnosed and cyproterone acetate was suspected as a precipitating factor for HHS. Cyproterone acetate was stopped and the patient was treated with hydration and insulin. This case highlights the hyperglycemic potential of cyproterone acetate and illustrate the importance of strict medical supervision, especially in patients with diabetes.