In the classic crossmatching assay the pre-existing antibodies against T-lymphocytes in lieu of donor kidney tissue are examined, the purpose of this study was to investigate the impact of transplantation on the result of post transplant cross matching in previously negative pairs, and clinical significance of this conversion. We enrolled 37 (26M/11F, 13-58 years) renal transplant recipients, all of them had received kidney from living related or unrelated donors, and immunosuppressives (steroid, cyclosporine, mycophenolate mofetil) were the same, cross matching performed one week before and two weeks after transplantation, in all instances we used fresh donor peripheral lymphocyte. All of these recipients had negative crossmatching before transplantation, but two weeks after, all second transplanted (4 patients), and two of the first transplanted patients had positive crossmatching, both of them were below 14 years old. One of the positive second transplanted patients lost his kidney due to acute rejection, we followed the remainder positive fore six months there were no any rejection episode in this group. We concluded that classic standard crossmatching is a reliable assay for detection of preformed antibody against donor antigens, but in high risk recipients (second transplantation, and children) we need methods to enhance the negative results of classic method.