Women treated with allogeneic immunization using paternal lymphocytes often request laboratory molecular tests using cell-free fetal DNA in maternal plasma. There is concern whether the treatment can interfere with its results. This study evaluated the applicability of fetal sex determination using fragments of sex-determining region Y (SRY) in the plasma of women submitted to paternal lymphocyte immunization.   Non-pregnant women blood samples were collected at two different moments: prior to paternal lymphocyte immunization and after three doses of the immunotherapy, in a prospective study. For women who became pregnant, another sample was collect during the first trimester. Amplification of the fragment of the Y chromosome (SRY) was performed using real-time PCR. The SRY gene was not identified in any of the plasma samples of the 50 non-pregnant women submitted to paternal lymphocyte immunization at either of the two moments evaluated. For the 26 pregnant women, the results of the identification of sex -determining in maternal plasma were completely in agreement with the infant sex. Paternal lymphocyte immunization does not affect the results of SRY fragment investigation in the plasma of women submitted to paternal lymphocyte immunization therapy.67231-4