Recently, a descriptive study suggested that diabetes and/or acute uncontrolled hyperglycaemia (defined as blood glucose measurements >10 mmol/l twice within any 24 h period) were associated with an increased length of hospital stay and higher mortality due to COVID-19 [13]. Furthermore, well-controlled blood glucose (glycaemic variability within 3.9–10.0 mmol/l) was reportedly associated with markedly lower mortality compared with individuals with poorly controlled blood glucose (upper limit of glycaemic variability exceeding 10.0 mmol/l) in patients with pre-existing type 2 diabetes during hospitalisation for COVID-19 [14]. However, direct correlation between fasting blood glucose (FBG) level at admission and clinical outcomes of COVID-19 patients without diagnosed diabetes has not been well established. Therefore, in this study, we examined the association between FBG on admission and the 28-day mortality of COVID-19 patients without previously diagnosed diabetes in two hospitals.