BACKGROUND: Thrombotic thrombocytopenic purpura (TTP) is a microangiopathic hemolytic anemia associated with ADAMTS-13 deficiency, a cleaving protease of von Willebrand factor (vWF). According to the literature, blood group O tends to be less common among these patients than in the general population. This study aimed to investigate whether the decreasing trend of blood group O in thrombotic thrombocytopenic purpura patients is observed in a Turkish cohort and to analyze the relationship between clinical outcomes and blood groups. PATIENTS AND METHODS: A total of 65 patients with acquired and five patients with congenital thrombotic thrombocytopenic purpura from two university hospitals were enrolled in this study. As a control group, the blood group data of 136,231 individuals who were not diagnosed without anemia were obtained from the archives of the Istanbul Medical Faculty Blood Centre. The blood groups were compared between cases and the Control Group using the chi-square test. Subsequently, the clinical outcomes of patients and categorized blood groups were compared by the chi-square test, Mann Whitney U test and Cox regression with Kaplan Meier analysis. RESULTS: This study shows that the decreasing trend of blood group O was not observed in this Turkish cohort. Regarding the relationship between blood groups and clinical outcomes, the AB blood group is associated with a good prognosis and blood group O is associated with a poor prognosis. In addition, relapses were more common with blood group A patients but less common in blood group B. CONCLUSION: The current study shows the association between thrombotic thrombocytopenic purpura and blood groups in the Turkish cohort. This study also contributes by analyzing the relationship between blood groups and clinical outcomes.