Endothelin-1 (ET-1) is a potent vasoconstrictor produced by endothelial cells and cleared from circulating blood mainly in the pulmonary vasculature. In a healthy pulmonary circulation, the rate of local production of ET-1 is less than its rate of clearance. In the present study we aimed to investigate whether abnormal pulmonary circulatory handling of ET-1 relates to poor clinical outcomes in patients with COVID-19-induced ARDS. Plasma ET-1 levels were measured in 18 mechanically ventilated COVID-19-induced ARDS patients, with simultaneous central venous and systemic arterial blood sampling on Days 1 and 3 following ICU admission. Two age- and sex-matched non-COVID-19 control groups were also used [mechanically ventilated non-COVID-19 critically ill and ARDS patients]. On ICU admission, COVID-19-induced ARDS patients had higher systemic arterial and venous ET-1 levels compared to non-COVID-19 ARDS and critically ill patients (p< 0.05). The arterial:venous (A:V) ET-1 ratio was higher in the non-COVID-19 ARDS patients [1.06 (0.93-1.20)] compared to the other two groups (p< 0.05). The A:V ratio was 0.63 (0.49-1.02) in the COVID-19-induced ARDS patients and 0.79 (0.52-1.11) in the non-COVID-19 critically ill patients. On Day 3, the A:V ratio in all three groups was < 1. The COVID-19 patient group was then divided based on 28-day ICU mortality. Although arterial and venous levels did not differ, the A:V ratio was statistically significantly higher on ICU admission in the non-survivors [0.95 (0.78-1.34)] vs 0.57 (0.48-0.92), p= 0.027]. Measurement of ET-1 clearance via A:V plasma ET-1 levels on ICU admission may assist in predicting outcomes in COVID-19 critically ill patients.