Red blood cell (RBC) transfusions are widely used in the management of patients with sickle cell disease (SCD). However, repeated RBC transfusions are often complicated by RBC alloimmunization. To investigate whether the frequency of RBC alloimmunization could be accounted for by racial and RBC phenotype differences between donors and recipients in Brazil, in this study we compared the RBC phenotype of 100 SCD patients with that observed in 120 randomly selected blood donors. A comparison of the RBC phenotype between the two groups revealed a statistically significant increase in the frequency of the C antigen in the donor population (P < 0.01), but no significant difference was observed for the A, B, D, c, E, e, K, k, Fy^a, M, N, S, s, and Jk^a antigens. Using standard techniques (indirect antiglobulin test, enzyme treatment, and low‐ionic‐strength solution) we observed an RBC alloimmunization rate of 12.9% (11/85) in the SCD patients. Fifteen alloantibodies were detected in 11 patients, and most (80%) involved antigens in the Rhesus and Kell systems. This observed RBC alloimmunization rate in SCD patients in Brazil is lower than that reported by studies from North America, suggesting that the requirement for extended antigen‐matched RBC transfusion for SCD patients in the setting of a RBC phenotype concordant donor‐recipient population may not be cost‐effective in some countries. © 1996 Wiley‐Liss, Inc.