Introduction: Blood donation saves lives. Blood safety is thus the most important factor throughout the whole process. Demographic location accounts for specific reasons for deferral. Living in high Andean areas causes adaptive hematological changes. Therefore, it is important to be aware of the reasons for deferral with a view to reorienting blood donation promotion programs. Objective: Determine the reasons for blood donor deferral in a Southern Peruvian hospital 3 935 m amsl. Methods: A retrospective cross-sectional study was conducted of the reasons for deferral of potential blood donors in EsSalud III Hospital in Juliaca. An analysis was performed of the records of pre-donors deferred from January 2014 to December 2018. The data were processed with the software STATA version 14. Results: Of the potential pre-donors selected (4 057), 1 699 (41.87%) were women and 2 358 (58.13%) were men. A total 1 458 (41.87%) were deferred. Men had the highest deferral rate with 882 (60.49%), whereas the rate for women was 576 (39.51%). Polyerythrocythemia was the most common reason for deferral with 669 (45.8%). Conclusions: The deferral rate was moderate (35.93%) and the main reason was high hematocrit (68.4%), followed by low hematocrit (6.8%).