Hemophilic dogs have an identical inheritance pattern and bleeding symptoms to human hemophilia A and therefore are a most suitable animal model of the human disease. The probability based on the pedigree data and the measurement of factor VIII coagulant activity (FVIII : C) have been used for carrier detection of canine hemophilia A. In order to improve the precision of carrier diagnosis, a ratio of FVIII : C to von Willebrand factor antigen (VWF : Ag) was used for the assessment of carrier status of canine hemophilia A. Two beagle dogs suspected to be hemophilic carriers by the reduction of FVIII : C to around 50 U/dl were tested for the ratio of FVIII : C/VWF : Ag. They were mated with a normal male beagle, and newborn puppies were tested for coagulation assays. One dog strongly suspected of being a carrier by the ratio was confirmed as a definite carrier by the production of a hemophilic puppy, which may be the first hemophilic dog in Japan resulting from a planned mating. The other dog suspected not to be a carrier by the ratio has never produced any hemophilic puppies. These findings suggest that the FVIII : C/VWF : Ag assessment is more reliable than FVIII : C alone for carrier detection of canine hemophilia A and is useful in the establishment of a canine hemophilic colony.