Background : Our objectives were to investigate the incidence of elevated methemoglobin level among COVID-19 patients at intensive care unit. The correlation of methaemoglobinemia with mortality and some haematological parameters was also tested. Methods: The diagosis of coronavirus -19 infection was confirmed by RT-PCR. The quantitative method for determination of methemoglobin was it’s cyanide derivative by spectrophotometry. The reference range was less than 2 percent. Results: There were altogether 46 patients (11 male, 35 female) included. Their median age value was 70 y. ( 29 – 89). The methemoglobin median value was 4,3%. 15 of 46 patients died. The methemoglobin median value of departed patients was 8 % , and 2,5% was among survivors. ( P= 0,001) 19 patients were blood transfused. Their methemoglobin median was 11 % , otherwise the non transfused patients presented 2,7 % methemoglobin median. ( p= 0,001). We performed two binary logistic regression calculation, in order to judge the elevated methemoglobin level as an independent risk factor for the mortality. The predictor of methaemoglobin was 0,062 the constant -1,266 the odds ratio 1,06. The other binary logistic regression tested the fact of transfusion for the mortality. Predictor of transfusion 1,1474, constant -1,2527 odds ratio 3,15 There was no significant correlation between methemoglobin and CRP level. Discussion: The methemoglobin is not able to transport oxygen. It’s association with mortality is discussed by some papers.There are several hypothesis in the literature to explain it’s occurrence in COVID-19. The role of coronavirus proteins as oxydative agents was mentioned. The importance of enzyme defect is emphasized. The impact of reactive oxygen free radicals in inflammation is also probable. Some papers mentioned applying local anesthetics and azythromycin as a risk. The role of transfusion is obscure, because the methemoglobin can be elevated in blood conserves during storage. Conclusion: Testing the methemoglobin seems to be important, but the pathomechanism needs further research.. The traceability and standardization of different measurement methods at intensive care units is the key for defining it’s pathogenetic role.