We studied 20 patients with a history of multiple pregnancy losses who had more than one partner for differences in reproductive efficiency with different partners. On the basis of obstetric history, patients were classified as primary (no children) or secondary (abortions after having children or stillbirths) aborters with their mates. Of 16 women diagnosed as primary aborters, 5 were primary aborters with their first mates and 11 with their second; one became a secondary aborter with her second partner. Five women were secondary aborters with either mate, and one of these was a secondary aborter with both. In no instance did a secondary aborter become a normally reproducing woman. These findings indicate that pregnancy failure in primary aborters may be due to compatibility between mating partners and that secondary aborters seem to manifest an innate aberration.