INTRODUCTION: We explored transmission of the coronavirus disease 2019 (COVID-19) in severely ill patients and analysed the relationship between co-morbidity and mortality or the need for intensive care unit (ICU) care. METHODS: Clinical data, treatment and outcome were analysed in this retrospective study of 101 consecutive patients with COVID-19 admitted to a regional Danish hospital from 2 March 2020, based on data from electronic medical records. RESULTS: The mean age was 71.8 years, 33% were never smokers and 82% had one or more predefined chronic diseases. In-hospital mortality was 30%, and 20% of the patients were offered ICU care. In ICU patients, we found a male preponderance (88% versus 44%, p = 0.006), but death (50% versus 25%, p = 0.053) and other pre-defined co-morbidities did not differ significantly from non-ICU patients. The source of infection was unknown in 74% of patients, related to endemic travel in 10%, hospital acquired in 6% and related to close acquaintances in 11%. COVID-19-related symptoms were initially observed from February 21 (week 8 and week 9) in the first three patients who had no known source of infection. We found that 7% of cases had an increased risk of in-hospital transmission, based on a 7-16 days delay in coronavirus testing. CONCLUSIONS: The frequency of co-morbidity in hospital-admitted COVID-19 patients and the correlation to death and ICU attendance were analysed. In all, 74% of the infection cases were of unknown source during the first weeks of the epidemic, which points to considerable community transmission and possibly pre-or asymptomatic transmission, also several weeks before 21 February 2020. FUNDING: none. TRIAL REGISTRATION: not relevant after correspondence with the Ethics Committee of Region Zealand. Furthermore, permission was granted from The Danish Data Protection Agency, Region Zealand (REG-070-2020).