During the COVID-19 pandemic, several hospital systems observed a reduction in patients with respiratory complaints. Using the Danish national registers, we conducted an observational study on disease severity and 30-day all-cause mortality for acutely admitted pneumonia patients before (3/19-3/20) and during (3/20-2/21) the pandemic. We calculated mortality rate ratios and Cox regression analyses. We identified 54,405 patients and during the pandemic, patients were older, more likely to be male, had more co-morbidity and a lower albumin on admission. Crude mortality was higher during the pandemic (8.4 vs. 6.9%). Adjusted hazard ratio for 30-day all-cause mortality was 1.07 (95%CI 1.01-1.14). We showed a small but significant, increase in mortality risk for patients admitted to hospital during the COVID-19 pandemic in Denmark.