We assessed the prevalence of unrecognized diabetes mellitus (DM) in patients with acute coronary syndrome (ACS) as determined by elevated glycosylated hemoglobin (HbA1c), fasting plasma glucose (FPG), and random plasma glucose (RPG) levels. This prospective study recruited 583 patients admitted with ACS without previous diagnosis of DM during 1-year period. Glycosylated hemoglobin was checked for most patients especially those with high values of FPG and or RPG. Patients were classified according to their glycemic state into 123 (21.1%) with DM, 82 (14.1%) with prediabetes, and 57(9.8%) with stress hyperglycemia, while 321 (55%) were classified as nondiabetics. Glycosylated hemoglobin estimation in the setting of ACS was helpful in the diagnosis of DM to eliminate the effect of stress-induced hyperglycemia that might accompany this condition.