The results study of renal allograft performed in the France-Transplant network, shows the beneficial effect of blood transfusions on graft survival; their immuno-suppressive effect allowed the tolerance of the transplant, despite of the presence, at times, of incompatibilities HLA. In this work, we have studied the effect of HLA-A, B, DR incompatibilities in 803 unrelated recipients according to their preimmunized status. In the 303 presensitized patients (by transfusion or transplantation) we show clearly the strong effect of the HLA compatibility and the additive effect of the three loci A, B, DR (94% of graft survival at two years for the best matched v.s. 36% for the worst). In the 500 non responder patients, there is no effect of the compatibility A, B, DR (69% for the best matched v.s. 71% for the worst). Presensitized patients reach currently 50% of the waiting list of dialysed. Cooperative efforts should avoid heavily mismatched transplants in case of preimmunization.