Objective: During the coronavirus disease 2019 (COVID-19) pandemic, baseline demographics and comorbidities of patients with COVID-19 have been presented, but there are limited data on outcomes of severely ill patients. We aimed to examine the association between patient characteristics and 30-day mortality among patients with COVID-19 treated in the intensive care unit (ICU). Design: Population-based cohort study. Setting: ICUs in Sweden. Participants: All consecutive patients with COVID-19 admitted to Swedish ICUs from 6 March to 5 April 2020. Main outcome measures: The primary outcome was 30-day mortality after ICU admission. Patient demographics, comorbidities and clinical characteristics were also retrieved. Results: A total of 604 patients were included. The median age was 61 years (interquartile range [IQR], 52-70 years) and 458 patients (76%) were males. The most common comorbidities were hypertension (35.9%) and diabetes (25.7%), whereas 36.4% of patients had no comorbidities. Median Simplified Acute Physiology Score (SAPS) 3 was 53 (IQR, 46-60). Of 573 patients with available respiratory support data, 487 (85.0%) received invasive mechanical ventilation. Among 518 patients with available data, 117 (22.6%) received renal replacement therapy. Median length of stay was 13 days (IQR, 6-20 days). Mortality at 30 days was 32.6%. In the multivariable Cox regression model, age (hazard ratio [HR] 1.06; 95% CI, 1.04-1.07 per year), the presence of one or more comorbidities (HR, 1.80; 95% CI, 1.20-2.68), chronic obstructive pulmonary disease or asthma (HR, 1.68; 95% CI, 1.12-2.50), hypertension (HR, 1.41; 95% CI, 1.01-1.99), and acute illness severity (SAPS 3 excluding age and comorbidity) (HR, 1.06; 95% CI, 1.04-1.09) were associated with 30-day mortality. Conclusions: This population-based cohort study presents 30-day mortality of 604 ICU patients with COVID-19. The higher mortality was explained by older age, the presence chronic illness, and acute illness severity.