Stress Hyperglycaemia (SH) evolving especially in the acute period of diseases and in post-traumatic patients is appeared in many diabetic and non-diabetic patients. Patients with undiagnosed diabetes mellitus may also present with complications of diabetes, with 5% fatal courses. The aim of this study was to determine the incidence of diabetes mellitus in patients who applied to Emergency Department (ED) for various reasons. The files of the patients were retrospectively examined. The patients were divided into the following groups: Group 0: No prior diagnosis of diabetes. Group 1: Stress hyperglycaemia. Group 2: Diabetes mellitus. Group 3: The patients had not been diagnosed with diabetes mellitus before visiting the emergency department, and they were not taking any treatment for diabetes. Group 4: Newly diagnosed diabetes mellitus. Group 2 had the highest number of patients (64.4%), followed by Group 3 (10.4%), Group 4 (4.3%), and Group 0 (4.1%). The highest average blood glucose values were found in Group 2 (258.00), followed by Group 3 (230.00), and Group 4 (237.00). Undiagnosed patients with diabetes mellitus risk can apply with complications, and this not only threatens the lives of these patients but also dramatically reduces their quality of their life. It should be considered that there could be diabetes mellitus patients, as well as stress hyperglycaemia patients, with a high blood sugar level, in the emergency department. Thus, we believe that patients diagnosed with a high blood sugar level should be advised to attend their family practices and/or a polyclinic for disease monitoring, with follow-up in an endocrine unit.