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Background and Objectives: A high proportion of suspected weak D patients referred to Héma‐Québec were genotyped as weak D type 42 (368/2105, 17.5%). These patients are currently considered D with regard to RhD immunoprophylaxis in pregnancy and transfusion. The goal of this study was to retrospectively evaluate the risk of alloimmunization in weak D type 42 patients and to characterize their RhD surface molecule expression on red blood cells (RBCs) in comparison to other weak D types (1, 2 and 3). Materials and Methods: A retrospective analysis using the weak D type 42 patients' medical data to verify potential anti‐D alloimmunization events was conducted. Quantitative analyses using flow cytometry were also performed on RBCs to quantify the cell surface density of the D antigen. Results: Data on 215 subjects with weak D type 42 were reviewed. None developed immune allo‐anti‐D; three had definite exposure to D+ red cells and 41 had possible exposure through pregnancy. Flow cytometry analysis showed that weak D types 1, 2, 3 and 42 had relative antigen densities of 2.7%, 2.2%, 8.1% and 3.6%, respectively, with R1R2 red cells referencing 100% density. The estimated antigen density range of weak D type 42 was 819–1104 sites per RBC. Conclusion: Our retrospective alloimmunization data analysis and antigen density study establish a basis for the consideration of a weak D type 42 individual as D+. This consideration would allow for a targeted reduction of RhD immunoprophylaxis in pregnancy and the unjustified use of D– units for transfusion. [ABSTRACT FROM AUTHOR]