This case series aimed to describe the clinical characteristics of severely or critically ill patients with COVID-19 and compare the clinical characteristics of patients who received invasive respiratory support with those of patients who received noninvasive respiratory support.We included all confirmed severe or critical illness cases of COVID-19 admitted to the Intensive Care Unit(ICU)of Zhongnan Hospital of Wuhan University,a COVID-19-designated hospital,from January 8 to March 12,2020.Cases were analyzed for epidemiological,demographic,clinical,APACHE II,SOFA,radiological features and laboratory data.Outcomes of all patients were followed up as of March 12,2020.This newly emerging virus had caused 55 confirmed severe or critical illness cases in ICU of a COVID-19-designated hospital.Most of the infected patients were men;more than half had underlying diseases,including hypertension,coronary artery disease and diabetes.The median age was 63 years old.Common symptoms at onset of illness were fever,fatigue and dry cough.Five(9.1％)hospitalized patients were presumed to have been infected in the hospital,and 4(7.3％)health care workers were infected in their work.Of the 55 confirmed severe or critical illness cases,10(18.2％)patients died during the follow-up period as of March 12 with the median follow-up period of 28 days(interquartile range 16-35).Nine patients received VV-ECMO for severe respiratory failure and 4(44.4％)patients died.Moreover,28 patients received invasive respiratory support and 14(50.0％)patients died.In this single-center study,55 severely or critically ill ICU patients were confirmed to have COVID-19 in Wuhan and the overall mortality was 29.1％.Totally 28(50.9％)of severely or critically ill ICU patients received invasive respiratory support and 14(50.0％)died during the follow-up period.