Aim: The study aimed to investigate risk factors affecting the transmission of and mortality from COVID-19 in patients diagnosed with a malignant disease. In this context, ABO blood groups, gender, age, type of malignant disease, type of anti-tumoral agents, comorbidities and stage were examined. Materials and Methods: Files of 1,256 patients who presented to our clinic between March and December 2020 were retrospectively reviewed. Patients diagnosed with a malignant disease who became infected with COVID-19 and those who did not were compared with regard to demographic, clinical characteristics and laboratory results (status of having been infected with COVID-19, ABO blood groups). Results: Of 1256 patients in total, 72 (5.7%) were diagnosed with COVID-19. Median age of cancer patients infected with COVID-19 was 53 years (18-80). The most common types of cancer included gastrointestinal cancer (22.2%), breast cancer (20.8%), genitourinary cancer (20.8%) and lung cancer (16.7%). Of the patients diagnosed with COVID-19, 18.1% (n=13) died. Multivariate analysis identified disease stage as an independent prognostic factor for the risk of mortality HR: 0.07, 95% CI (0.007-0.74), (p=0.02). A comparison of patients who became infected with COVID-19 and those who did not with regard to ABO blood groups (p=0.39) showed no statistically significant differences between the two groups. There was also no correlation between ABO blood groups and the risk of COVID-19-related mortality (p=0.83). Conclusion: In patients suffering from malignant diseases, the ABO blood type exhibited no correlation with the risk of COVID-19 transmission and mortality. This study determined the presence of metastatic disease as a negative prognostic factor. Patients suffering from a metastatic malignant disease represent a high risk group for COVID-19 and should be treated using all necessary precautions.