Hyperglycaemia is common in patients with community-acquired pneumonia (CAP) and is a predictor of severe outcomes. Data are scarce regarding whether this association is affected by diabetes mellitus (DM) and also regarding its importance for severe outcomes in hospital. We determined the impact of blood glucose on severe outcomes of CAP in hospital. We studied 1318 adult CAP patients hospitalised at three Danish hospitals. The association between blood glucose and DM status and severe clinical outcome (admission to an intensive care unit (ICU) and/or in-hospital mortality) was assessed by logistic regression. Models were adjusted for CURB-65 score and comorbidities. 12% of patients had DM. In patients without DM an increase in admission blood glucose was associated with risk for ICU admittance (OR 1.25, 95% CI 1.13-1.39), but not significantly associated with in-hospital mortality (OR 1.10, 95% CI 0.99-1.23). In patients with DM an increase in admission blood glucose was not associated with ICU admittance (OR 1.05, 95% CI 1.00-1.12) or in-hospital mortality (OR 1.05, 95% CI 0.99-1.12). An increase in admission blood glucose (only in patients without DM) was associated with a higher risk for ICU admittance and a trend towards higher in-hospital mortality. DM was not associated with a more severe outcome of CAP.