The aim of present study was to determine the efficacy of immunotherapy with paternal lymphocytes for unexplained infertility. It has been apparent that reproductive success may be affected by the presence of abnormal autoantibodies. Unexplained infertility and repeated pregnancy wastage has been reported in the presence of abnormal autoantibodies. These data suffest that abnormal immune function may be an important pathologic entity contributing subfertility in patients with unexplained infertility. Therefore, immunotherapy may be a possible treatment modality for patients with unexplained infertility. Some investigators have reported that a proportion of infertile couples with repeatedly unsuccessful ET showed close histocompatibility similar to those of spontaneous recurrent abortion. Recently, it has been noted that immunotherapy with paternal lymphocytes achieves a high efficacy in preventing subsequent abortion in women with primary recurrent abortion of unknown cause, which was mediated by immune reaction including blocking antibody. To substantiate the hypothesis, we applied immunotherapy preceding Peritoneal Oocyte and Sperme transfer (POST) to 43 patients, 47 cycles of 82 patients, 89 cycles with at least three previous IUI failure from April, 1993 to February, 1995. There were no significant differences between treatment and control group in clinical response and hormonal response to controlled hyperstimulation. there was no significant difference between treatment and control group in pregnant rates per cycles (42.6% versus 28.6%), but a significantly lower abortion rate per pregnancy in treatment group, with 10.0% (2/20) compared with 50.03% (6/12) in control group. This study may suggest that immune therapy for patients with unexplained infertility with paternal lymphocytes might be beneficial.