The etiology of preeclampsia (PE) is so far unknown; nevertheless both clinical and experimental findings suggest the possibility that immunogenetic factors operate in this disorder. In order to verify the role of immunogenetic factors in PE, a selected group of women (n = 26) affected with PE was chosen and HLA (human leucocytes A) frequency distribution, HLA antigens sharing between couples, homozygosity, incidence of HLA phenotypes and maternal antibody production were investigated. Neither significant differences in HLA frequencies nor homozygosity condition in preeclamptic and/or in the partners have been noted. HLA typing of the couples under study however demonstrates a very high antigen sharing between partners if compared with the sharing found in couples with normal reproductive performance. In 46% of PE women it was possible to demonstrate the presence of cytotoxic antibodies against surface structures of partner lymphocytes. The role of the MHC (major histocompatibility complex) in fetal survival and outcome is discussed.