Introduction: Focus of this study was to assess leukocyte telomere length change in mild to moderate COVID-19 patients and concomitant influence of inflammation, oxidative stress (OS), pulmonary involvement and implemented therapy on the course of the disease.    
Material and methods: Routine biochemical/haematological parameters, markers of OS (prooxidants and antioxidants), vitamin D, IgM and IgG antibodies level and relative length of leukocyte telomeres (rLTL) were measured at diagnosis, after 14 and 21 days from the disease onset in blood samples of 31 consecutive COVID-19 patients, treated on an outpatient basis.
Results: Although the patients had reduced rLTL at baseline (median: 0.592; 25th ? 75th percentiles: 0.518-0.724), it significantly increased during the follow-up (median: 0.773; 25th ? 75th percentiles: 0.615-0.923; P<0.01). The rate of telomere attrition was associated with the extent of OS and pulmonary involvement. During follow-up, the burden of OS was reduced, while antioxidative defence mechanisms were recovered. The use of antibiotics and N-acetylcysteine-propolis supplementation was associated with telomere lengthening.
Conclusion: Results of this study revealed significant interaction between OS, inflammation and leukocyte telomere length attrition in COVID-19. Our data suggest that rRTL can be a biomarker that enables more precise therapy decision and accurate patient status estimation.