The COronaVirus DISease 19 (COVID-19) is a pandemic infectious disease caused by the novel coronavirus Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV-2). Older age and presence of comorbidities, including diabetes, were shown to be associated with a more severe course and a higher fatality rate. Studies from the most affected countries, including China, United States and Italy, seem to indicate that prevalence of diabetes among patients affected by COVID-19 is not higher than that observed in the general population, thus suggesting that diabetes is not a risk factor for SARS-CoV-2 infection. However, a large body of evidence demonstrate that diabetes is a risk factor for disease progression towards critical illness, development of acute respiratory distress syndrome, need for mechanical ventilation or admission to intensive care unit, and ultimately death. The mechanisms underlying the relationship between COVID-19 and diabetes remain to be elucidated. In particular, it is still unresolved whether is diabetes per se, especially if poorly controlled, or rather the various comorbidities/complications associated with it that predispose patients with COVID-19 to a worse prognosis. In fact, conditions that cluster with diabetes in the context of the metabolic syndrome, such as obesity and hypertension, or complicate chronic hyperglycemia, such as cardiovascular disease and chronic kidney disease, have also been associated with poor prognosis in these individuals and the available studies have not consistently shown that diabetes predict disease severity independently of them. Electronic supplementary material The online version of this article (10.1007/s00592-020-01586-6) contains supplementary material, which is available to authorized users.