Diabetes mellitus is associated with acute and chronic complications that cause major morbidity and significant mortality. We report a 69-year-old man with unknown diabetes, presenting vague epigastric discomfort, polyuria, polydipsia, fatigue, anorexia, weight loss over 1 week and severe chest pain for 1 day. Electrocardiogram revealed ST-segment elevation in lead V1 through V6. Blood chemistry examination revealed a creatine kinase level of 2053 U/l, creatine kinase-MB (CK-MB) level 43 U/l, a troponin I level of 23.21 ng/ml, a blood sugar level of 957 mg/dl, blood osmolality of 324 mosm/kg and no ketonemia. The patient was diagnosed as hyperosmolar hyperglycemic state accompanying acute anterior wall ST-segment elevation myocardial infarction on unknown diabetes mellitus. Aggressive therapy failed to ameliorate the patient's clinical outcome.