OBJECTIVES: To evaluate the relationship between admitting (non-fasting) blood glucose and in-hospital mortality in patients with and without diabetes mellitus presenting with acute coronary syndrome (ACS) in Oman. METHODS: Data were analyzed from 1551 consecutive patients admitted to 15 hospitals throughout Oman with the final diagnosis of ACS during January to June 2007, as part of Gulf RACE (Registry of Acute Coronary Events). Admitting blood glucose was divided into four groups; namely, euglycemia (≤7 mmol/l), mild hyperglycemia (>7 to <9 mmol/l), moderate hyperglycemia (≥9 to <11 mmol/l), and severe hyperglycemia (≥11 mmol/l). Analyses were performed using descriptive and multivariate statistical techniques. RESULTS: Thirty-eight percent (n=584) and 62% (n=967) of the patients were documented with and without a history of diabetes mellitus, respectively. In non-diabetic ACS patients, there was a near-linear relationship between admitting blood glucose and in-hospital mortality. Non-diabetic patients with severe hyperglycemia were associated with significantly higher in-hospital mortality compared to those with euglycemia (13.1% versus 1.52%; p<0.001), mild hyperglycemia (13.1% versus 3.62%; p=0.003) and even moderate hyperglycemia (13.1% versus 4.17%; p=0.034). Even after multivariate adjustment, severe hyperglycemia was still associated with higher in-hospital mortality when compared to both euglycemia (odds ratio (OR), 6.3; p<0.001) and mild hyperglycemia (OR, 3.42; p=0.011). No significant relationship was noted between admitting blood glucose and in-hospital mortality among diabetic ACS patients even after multivariable adjustment (all p-values >0.05). CONCLUSIONS: Admission hyperglycemia is common in ACS patients and is associated with higher in-hospital mortality among those patients with previously unreported diabetes mellitus.