The retrospective study of patients with COVID-19 coronavirus infection identifies possible causes of fluid accumulation in various serous cavities and compares the presence of fluid accumulation syndrome in cavities with the severity of the course of coronavirus infection. A total of 85 charts of inpatients treated at the Kabanov A.N. Municipal Clinical Hospital No. 1 for confirmed coronavirus infection were studied. They were divided into 2 groups: 1st – having a syndrome of effusion in one or more cavities (pleuritis, pericarditis and/or ascites): 48 patients; 2nd group ̵ not having pleural effusion syndrome: 37 patients. The data obtained in the study showed that the causes of fluid accumulation in serous cavities in patients with COVID-19 are hypoproteinemia and concomitant diseases running with general edema, more often it is chronic kidney disease stage 3 or more, liver cirrhosis, malignant tumors of different localization. Taking into account that vital function indices were comparable in the studied groups, we assumed that pleural effusion syndrome in COVID-19 patients in general does not aggravate the course of the disease, except for rare isolated cases with massive effusions into the pleural or pericardial cavity. Taking into account the obtained results, attention to early correction of hypoproteinemia and active detection of concomitant pathology in COVID-19 patients with cavity fluid accumulation syndrome is justified.