Background: In critically ill patients, hyperglycemia is common and frequently multifactorial. Severe hyperglycemia has been connected to negative outcomes in several conditions in non-diabetic patients. It can cause dysfunction of the endothelial system, release of cytokines, activation of platelets, dysfunction of the mitochondria, and disturbances in electrolytes and acid base balance. Diabetic patients have not shown these associations. Aim of the Work: to study the effect of hyperglycemia in critically ill patients diagnosed with diabetes mellitus regarding the length of ICU or hospital stay and incidence of complications. Patients and Methods: The current Prospective observational cohort study was conducted at hospitals of Ain-Shams University and the El Zeitoun Specialized Hospital. Sixty patients, all older than eighteen, who had been admitted to the intensive care unit due to sepsis and were known to have diabetes were enrolled in the study. Two groups of patients were randomly assigned: the first group's random blood glucose (RBG) measures were less than or equal to 150 mg/dl, and the second group's RBG measurements were greater than 150 mg/dl. Patients were assessed for clinical and laboratory parameters. Results: patients in the second group, who had RBG level above 150mg/dl, had a significantly higher Acute Physiology and Chronic Health Evaluation (APACHEII) than the other group. Also, patients in the second group had a significantly longer hospital stay. On the other hand, both groups were comparable in sex, age, body mass index as well as special habits as smoking. The development of complications during ICU stay was similar in both groups. Conclusion: Glycemic control in the ICU is a very import part in the management of ICU patients. Hyperglycemia is associated with higher APACHEII score and longer intensive care unit stay.