SummaryThe purpose of the investigation was to study whether blood‐group incompatibility between partners is of importance as a causal factor in disturbed fertility.The material comprises:385 marriages in which all–and at least four–pregnancies had terminated with live births.269 marriages in which, for unknown reasons, at least two pregnancies terminated in intrauterine death of the foetus and abortion.83 marriages, childless for unknown reasons.No difference was found between the three series with respect to the frequency ofABOandRhincompatibility. Nor did the observed distribution of mating classes forMN, Lewis blood groups or secretor and non‐secretor characteristics deviate from that expected.A statistically highly significant discrepancy was found in the intra‐uterine death series and subseries thereof: the frequency ofABOincompatible matings in mating class husbandRh+: wifeRh+ was about 50%, whereas in matings involving at least oneRh—partner the frequency ofABOincompatible matings was about 20 %. Some implications of this finding ar