Eighty-five couples with habitual abortion were studied: 43 were primary aborters (i.e., had no advanced pregnancy beyond 20 weeks gestation), and 42 were secondary aborters. The only significant finding in primary female aborters was a reduction in the incidence of B35. In contrast, in secondary female aborters there was a reduction in the incidence of DR3 and A1/B8/DR3 and an increase in the degree of homozygosity of HLA B. In the male partners of primary aborters, there was a reduced incidence of DR3 and A1/B8/DR3. In addition, in male partners of both primary and secondary aborters, there was increased HLA-B homozygosity and increased frequency of DR4-5 phenotype. Increased sharing of HLA antigens between partners was not found to be significantly different from the control population. These findings emphasize that primary and secondary female aborters form distinct populations, which could explain differences in immunological responses to various antigens, including fetal-related antigens. The closely related findings in female secondary aborters and male partners of primary aborters are intriguing but cannot be explained. Whether or not these genetic markers bear any relationship to reproductive success remains to be determined.