Abstract <p>The level of interleukin-17A (IL-17A) was studied in the blood of 82 patients with diabetes mellitus (DM) who had contracted coronavirus disease 2019 (COVID-19) 2–4 years ago. IL-17A was determined using enzyme immunoassay kits. It was shown that the concentration of the cytokine was significantly higher in diabetic patients and in people who had COVID-19. A difference in cytokine levels in the blood of patients who had mild and severe forms of COVID-19, as well as a positive effect of vaccination, was noted. IL-17A concentration increased with diabetes duration of more than 15 years, with body mass index more than 30&#xa0;kg/m² and glycated hemoglobin (Hb1Ac) &gt; 7.5%. COVID-19 additionally increased these indicators. There is a positive effect of metformin on IL-17A concentration in patients with diabetes and high interleukin response to COVID-19 in the absence of biguanide treatment. A positive effect of insulin and type 2 sodium-dependent glucose cotransporter inhibitors (iSGLT-2) was also noted. The amount of IL-17A increases in blood plasma with pathological changes in the left ventricular ejection fraction, glomerular filtration rate, and albuminuria. The study of the concentration of IL-17A in blood plasma depending on the age of patients showed a tendency for decrease in the amount of interleukin with age. A high level of IL-17A is observed in the blood plasma of patients with diabetes who had COVID-19 2–4 years ago comparable to an acute disease. In the absence of treatment of patients with diabetes with glucose-lowering drugs in COVID-19, the level of IL-17A reaches high values. The study of the concentration of IL-17A in blood plasma depending on the age of patients showed a tendency to a decrease in the amount of interleukin in older age groups.</p>