Patients with COVID-19 who require supplemental oxygen and/or mechanical ventilation are routinely treated with dexamethasone. However, glucocorticoids can exacerbate dysglycaemia, and the benefits of dexamethasone treatment in patients with diabetes mellitus were unclear. A retrospective analysis has assessed data from the first two waves of the COVID-19 pandemic in the UK. Mortality was reduced in the second wave compared with the first wave, with dexamethasone being independently associated with reduced risk of admission to the intensive care unit and/or death. Furthermore, a multivariate analysis demonstrated that the independent effect size of dexamethasone was similar for patients with and without diabetes mellitus. The authors conclude that dexamethasone is beneficial for patients with severe COVID-19 and diabetes mellitus, but that treatment guidelines need to incorporate strategies to identify and manage steroid-induced hyperglycaemia.