Background: A well-functioning blood transfusion service is essential for effective health-care delivery. Despite the fact that blood transfusions can save lives, the role of blood transfusion services is to assure the supply of safe blood from healthy donors. Although having a sufficient supply of blood is critical in blood transfusion, the caveat is that the collection and transfusion protocol must not put both the receiver and the donor in danger.Identifying and attempting to retain repeat blood donors is critical. Understanding the most typical reasons for their deferral will enable us to prevent or remedy them as soon as possible, hence increasing their retention.Objective: To determine the prevalence and causes of deferral among repeat blood donors in Dar es salaam, Tanzania. Methodology:A cross-sectional study of 170 repeat blood donors at the Muhimbili National Hospital blood donation unit and the National Blood Transfusion Service (NBTS) form Dec 2021 to Jan 2022. Using a designed questionnaire donor's socio and demographic characteristics and assessment of donor's health and risk factors using NBTS questionnaire version 8.0. Donors were also tested for hemoglobin level using Copper sulphate method, ELISA test for HBV, HCV, and HIV, and Rapid Syphilis test using data provided in a developed questionnaire. Data were recorded and analyzed using Statistical Package for Social Science (SPSS) statistics software version 23 and p-values less than 0.05 were considered statistically significant. Results:The men made up the majority of the 170 repeat blood donors, with an average age of 38 years. Seventeen percent of repeat blood donors were deferred. Temporary deferral was most commonly caused by low hemoglobin (41.4%), while permanent deferral was most commonly caused by the presence of TTIs (24.1 percent). Age above 45, female sex, and an inter-donation time of more than or equal to 5 months are all associated with deferral. Males were deferred in the majority due to TTIs (41%), whereas females were deferred in 75% due to low hemoglobin. Conclusion:Deferral is common among repeat blood donors, and the leading cause is low hemoglobin and TTI. Deferral is associated with age over 45, female sex, and an inter-donation period of more than or equivalent to 5 months.