Introduction: To analyze the prevalence of alloantibodies in multiply transfused patients. Methods: This study was a retrospective, exploratory and descriptive study with a quantitative approach. The study sample comprised 185 patients transfused at a referral service in the city of Passo Fundo, Rio Grande do Sul, from January 2016 to February 2018. Results: Overall, the antibodies identified were as follows: anti-E in 47 patients (18%), anti-D and anti-K in 28 patients each (11%), anti-C in 21 patients (8.1%), and inconclusive antibody results in 23 patients (8.9%). Females were a majority (55.7%), mean age was 48.8 years and mean quantity of blood transfused was 7.2 bags. Cardiovascular disorders were the most common comorbidities, in 39 patients (21.2%), followed by oncological disorders, in 38 patients (18.4%). Conclusion: Alloimmunization is an important and frequent clinical condition that increases the risk of hemolytic reactions and is associated with significant patient morbidity and mortality
