The accuracy of hemophilia A carrier detection during pregnancy has been determined using combined measurement of VIII:CAg and VIIIR:Ag. These immunoassays detect determinants that are sufficiently stable in plasma that the assays could be done on frozen samples that had been obtained when women were seen for antenatal diagnosis studies (carrier women) or for routine prenatal care (controls). A linear discriminant was calculated that best separated the data for 32 normal women and 25 obligate carriers of the hemophilia gene. Twenty-three of 25 carriers (92%) and all 32 control women were correctly identified in this analysis. The overall classification accuracy (55/57, 96%) is comparable to that obtained by VIII:C and VIIIR:Ag measurements using freshly drawn blood samples in nonpregnant individuals. This study demonstrates that hemophilia A carriers can be detected during pregnancy with sufficient accuracy that the information may be used for genetic counseling.