Background: Rh antibodies produced by patients receiving Rh-matched RBC units may be associated with inheritance of altered RH alleles or a result of altered Rh epitopes on donor red blood cells (RBCs). Based on this, our aim was to evaluate unexpected Rh antibodies in Brazilian patients receiving regular transfusions and access the clinical significance of the alloantibody produced. Material andmethods: We investigated 7 patients (5 with sickle cell disease (SCD), 1 with myelodysplastic syndrome (MDS) and 1 with β-thalassemia) with unexplained Rh antibodies. All patients had complete serological and molecular analyses. A lookback on the donor units transfused to those patients was performed and donors suspected of having Rh variants were recruited for further analysis. Laboratory and clinical findings were used to evaluate the clinical significance of the alloantibodies produced. Results: Unexpected Rh antibodies found in these patients were not linked to expression of partial Rh phenotypes according to serological and molecular analyses. Anti-D was found in 2 patients, anti-C was found in one patient, anti-c was found in one patient and anti-e was found in 3 patients carrying conventional D, C, c and e antigens respectively. Serological and molecular analyses on donors'samples revealed that 6 donors whose RBCs were transfused to these patients carried partial Rh antigens. Only one anti-e in a patient with β-thalassemia was autoreactive and could not be explained by RH diversity in his donors. Laboratory and clinical evidences of a delayed hemolytic transfusion or decreased survival of transfused RBCs were associated with 3 of 7 Rh antibodies at first detection. Discussion: Our study provides evidence that patients exposed to RBC units from donors with Rh variants may develop antibodies and some of these may be of clinical significance.