The role of histocompatibility antigens in pregnancy outcome is controversial. This controversy may be because the initial studies were of small numbers or the patient groups were not homogeneous. The purpose of this study is to clarify these discrepant results by carrying out histocompatibility antigen typing and mixed lymphocyte culture on couples with idiopathic recurrent spontaneous abortion and by comparing results with those of fertile couples. Sixty couples with at least three spontaneous abortions and 60 normal couples with at least two successful pregnancies were included. Histocompatibility antigen typing and mixed lymphocyte culture were performed by using the standard techniques. The data were analyzed statistically with the Fisher exact test and chi 2. Our results failed to show any difference between normal and aborting couples with regard to HLA-A, HLA-B, and HLA-DR distribution or sharing or to mixed lymphocyte culture responsiveness. Our study, along with other studies, emphasizes that the histocompatibility antigen system does not have an impact on pregnancy outcome.