Background. In 5–20 % cases, a new coronavirus infection (COVID-19) can cause pneumonia and require hospitalization. There is a high incidence of hypokalemia among hospitalized patients with COVID-19. Objective. To assess the frequency of hypokalemia among hospitalized patients with pneumonia caused by COVID-19 and its relationship with the severity of the disease. Design and methods. A cohort study included 43 patients with verified pneumonia caused by COVID-19. The level of potassium in venous blood was measured upon admission to the hospital. Differences in the levels of acute phase proteins, oxygen saturation, and the level of lung damage were compared between patients with and without hypokalemia. Results. The potassium level below 3,5 mmol/L was detected in 16 out of 43 patients (37,2 %), the median of potassium level in patients with hypokalemia was 2,9 (2,7–3,3) mmol/L. Patients with hypokalemia had a significantly lower oxygen saturation and higher percentage of lung tissue damage according to computed tomography: 91 (89–95) vs 96 (93–98)% (p = 0,001) and 64 (60–70) vs 48 (36–64)% (p = 0,019), respectively. Significantly higher levels of C-reactive protein and lactate dehydrogenase were observed in patients with hypokalemia. Conclusions. Our results showed a high incidence of hypokalemia in hospitalized patients with pneumonia caused by COVID-19. Hypokalemia was positively associated with higher percentage of lung tissue damage and the levels of acute phase proteins. © 2020 All-Russian Public Organization Antihypertensive League. All rights reserved.