• The primary objective of this study was to determine the relationship between pre-existing diabetes and in-hospital mortality of COVID-19 patients.

• This study was conducted in the Lombardy region of Italy and the data came from Policlinico di Monza hospital.

• 373 patients were assessed in this study.

• The mean age of the patients was 72 years and 69 of the 373 patients had pre-existing diabetes.

• It was found that patients with pre-existing diabetes were more commonly associated with hypertension and chronic kidney disease when comparing them to non-diabetic patients.

• It was also found that diabetes was independently associated with an increased in-hospital mortality while hypertension and chronic kidney disease were not.

• This study demonstrates that in the Lombardy region of Italy, patients with diabetes are at a higher risk of in-hospital mortality than non-diabetic patients.