IntroductionIn late 2019, a new virus severe acute respiratory syndrome coronavirus-2, which causes a new serious disease called coronavirus disease 2019 (COVID-19), spread quickly all over the world. Many complications of diabetes mellitus (DM) such as microvascular and macrovascular complications increase the risk for COVID-19-related severity and mortality.ObjectiveTo assess the effect of DM on outcome and severity of COVID-19.Patients and methodsThis work was a retrospective observation that included confirmed COVID-19-infected patients (PCR positive for COVID-19) who were admitted to Shebin El-Kom Teaching Hospital, Menoufia, Egypt, from March 2021 to March 2022. Laboratory investigations such as age, sex, and sings of severity of COVID-19 were studied. In this study, patients were divided into three main groups as follows: group 1 diabetic patients had known DM, group 2 (hyperglycemic group) patients were without past history of DM but were known to have hyperglycemia with COVID-19, and group 3 (control) had normal blood glucose.ResultsA total of 300 patients were divided into diabetic and hyperglycemic groups and were compared with the control group. It was found that diabetic and hyperglycemic groups were significantly associated with severe and critical disease, long hospital stay, increased number of admissions in critical care unit, increased number of patients under mechanical ventilation, and increased mortality rate from COVID-19 compared with the control group.ConclusionsPatients with DM are more liable for severe acute respiratory syndrome coronavirus-2 infection, ICU admission, and increase in the rate of death. In addition, new hyperglycemia, even without history of DM, increased the severity of COVID-19 infection. Special care should be given for diabetic patients. Therefore, glucose monitoring and tight control of blood glucose are essential for all.