Objective The aim of this study was to evaluate the control of hyperglycemia in diabetic patients by evaluating glycosylated hemoglobin A1c (HbA1c), to differentiate between already diabetic patients and stress-induced hyperglycemia in patients who presented with first-time hyperglycemia, and to study the effect of hyperglycemia on mortality. Background Hyperglycemia is common in critically ill patients and has poor impact on the outcome. Therefore, it should be discovered early and controlled by evaluating glycosylated HbA1c. Patients and methods This cross-sectional study included 281 patients attending the ICU of Mit Ghamr General Hospital, Dakahlia, Egypt, for a period of 1 year from May 2015 to May 2016. These patients were classified as diabetic patients, patients with first-time hyperglycemia, and nondiabetic patients. After obtaining written informed consent from patients or from relatives of comatose patients, all patients were evaluated clinically and by means of routine laboratory investigations including the following: random blood sugar, fasting blood sugar, HbA1c, complete blood count, prothrombin time, international normalized ratio, serum creatinine, blood urea, and serum albumin for all patients and computed tomography of the brain, arterial blood gases, and cardiac enzyme for selected cases. Results Hyperglycemic patients with higher HbA1c showed worst outcomes with increased mortality. Stroke, renal impairment, and higher HbA1c were independent risk factors for mortality in ICU patients. There was a significant positive relationship between HbA1c and white blood cells, blood urea, and creatinine. Moreover, there was a negative relationship between HbA1c and red blood cells, hemoglobin, and albumin. Conclusion Hyperglycemia is a common phenomenon in critically ill patients regardless of whether or not the patients were diabetic; the higher the HbA1c, the higher the incidence of mortality and complications.