We have performed class I HLA antigen testing in 42 women with recurrent habitual abortions and in their husbands. The main criterion for inclusion in this group was a frequency of more than two abortions without the known reasons for abortion and without a living child. 29 couples with a least two healthy children and no abortion in the clinical history served as a control group. The data were evaluated in respect to the association of HLA and disease as well as to the level of histocompatibility within the couples. In our results we did not find any significant association between HLA-A, B, C phenotypes and the habitual abortion. On the other hand, we observed a significantly higher level of histocompatibility in the couples with habitual abortions in comparison to the control couples. The frequency of identity in one and more histocompatibility antigens was in the patient group significantly higher than in the control group and as it could be expected by chance, calculated on the basis of antigen distribution within the loci A, B, C in the population in the southern part of the GDR. In no case we have revealed lymphocytotoxic antibodies as a possible cause for miscarriage. Our results are in favour of the hypothesis that a higher level of histocompatibility could be an immunological explanation for the habitual abortions. In addition, this hypothesis was supported by the successful immunotherapy with leukocytes derived from the husband or nonrelative donors.