Aim: It is known that blood groups and susceptibility to thrombosis are related to each other. COVID-19 is a disease with a high mortality rate with thrombotic events. In this study, we aimed to investigate the relationship between blood types and the severity of COVID-19 disease and in-hospital mortality. Methods: This study was carried out retrospectively using the data of PCR positive patients hospitalized for COVID-19 whose blood group was studied between 01.03.2020 and 01.06.2021. The patients were divided into four groups. Group 1 (control group) consisted of 37996 people whose blood group was studied within one year and 1437 outpatient PCR positive cases in Group 2, 1024 PCR positive hospitalized patients in Group 3, and 413 PCR positive patients hospitalized in the intensive care unit in Group 4 were presented. The groups were compared in terms of age, gender, AB0, and Rh blood groups. In addition, the data of the living and deceased patients in Group 4 were compared. Results: The rate of PCR positive test was higher in the A and AB blood groups (p:0.037, p<0.001, respectively), and lower in the O blood group (p<0.001). There was no significant difference between AB0 and Rh blood groups in the hospitalization and mortality in the intensive care unit. Conclusion: The present study found that blood group 0 may be protective against COVID-19, blood groups A and AB had a greater susceptibility to the disease, but blood group AB0 did not affect the course of the disease and was not associated with mortality.