Severe acute respiratory syndrome (SARS) has become a global public health emergency. To evaluate the characteristics and outcomes of patients with SARS in Hong Kong and to identify predictors of mortality. Retrospective cohort study. Quarantine hospital for patients with SARS in Hong Kong. 267 consecutive patients hospitalized from 26 February to 31 March 2003 for probable or confirmed SARS. Clinical, laboratory, and radiographic measures; 3-month mortality rate. According to our case definition, there were 227 cases of confirmed SARS and 40 cases of probable SARS. Common presenting symptoms were fever (99% of patients), chills (74%), malaise (63%), and myalgia (50%). Laboratory findings included lymphopenia (73%), thrombocytopenia (50%), hyponatremia (60%), and elevated levels of lactate dehydrogenase (47%) and C-reactive protein (75%). During hospitalization, incidence of diarrhea (53%), anemia (53%), and acute renal failure (6%) increased. Sixty-nine patients (26%) required intensive care because of respiratory failure. The 3-month mortality rate was 12% (95% CI, 8% to 16%). Factors contributing to mortality were respiratory failure, acute renal failure, and nosocomial sepsis. On multivariate Cox regression, age older than 60 years (relative risk, 5.10 CI, 2.30 to 11.31; P < 0.001) and lactate dehydrogenase level greater than 3.8 micro kat/L at presentation (relative risk, 2.20 CI, 1.03 to 4.71; P = 0.04) were independent predictors of mortality. Because of the longer follow-up period in our cohort, the mortality rate in these patients is higher than rates reported in previous studies. Advanced age and high lactate dehydrogenase level at presentation predict mortality. *For members of the Princess Margaret Hospital SARS Study Group, see the Appendix.