Hyperglycemia, hypoglycemia and blood glucose fluctuation are associated with the outcomes in critically ill patients, but target of blood glucose control is debatable especially in patients with diabetes regarding to the situation of blood glucose control before admission to ICU. This study is aimed to investigate the association between glycemic gap which is calculated as the mean blood glucose level during the first 7 days after admission to ICU minus the A1C-derived average glucose and outcomes of critically ill patients with diabetes.This study undertaken in two intensive care units (ICUs) with a total of 30 beds. Patients with diabetes expected to stay for more than 24hrs were enrolled, HbA1c was tested within 3 days after admission and converted to the A1C-derived average glucose (ADAG) by the equation: ADAG = [ ( HbA1c * 28.7 ) - 46.7 ] * 18-1, arterial blood glucose measurements were fourth per day routinely during the first 7 days after admission, APACHE II score within first 24 hours, the mean blood glucose level(MGL), standard deviation(SD), and coefficient of variation(CV) during first 7 days were calculated for each person, GAPadm and GAPmean were calculated as admission blood glucose and MGL minus ADAG respectively, the incidence of moderate hypoglycemia(MH) and severe hypoglycemia(SH), total dosage of glucocorticoids and average daily dosage of insulin within 7 days, duration of renal replacement therapy(RRT), ventilator-free hours, and non-ICU stay days within 28 days were also collected. Patients enrolled were divided into survival group and nonsurvival group according to survival or not at 28-day and 1-year after admission, exploration of the relationship between parameters derived from blood glucose and mortality in critically ill patients enrolled were undergone.502 patients were enrolled and divided into survival group (n=310) and nonsurvival group (n=192). It was shown that two groups had comparable level of HbA1c, the nonsurvivors had greater APACHE II, MGL, SD, CV, GAPadm , GAPmean , and higher hypoglycemia incidences. Less duration of ventilator-free, non-ICU stay, and longer duration of RRT were recorded in nonsurvival group, of whom received less carbohydrate intake, higher insulin daily dosage, and glucocorticoid dosage. GAPmean had the greatest predictive power with AUC of 0.820(95%CI: 0.781-0.850), the cut-off value was 3.60mmol/L(sensitivity 78.2% and specificity 77.3%). Patients with low GAPmean tended to survive longer than the high GAPmean group 1 year after admission.Glycemic GAP between the mean level of blood glucose within first 7 days after admission to ICU and A1C-derived average glucose was independently associated with 28-day mortality of critically ill patients with diabetes, the predictive power extended to 1 year. The incidence of hypoglycemia was associated with mortality either.This article is protected by copyright. All rights reserved.