In a patient with hemophilia A, ahaptoglobinemia was seen during the acute hemorrhagic stage; it must be considered a temporary phenomenon and not genetically determined. Chromosome analysis in the patient, his mother and sister, who were found to be carriers, revealed an X-chromosome with an enlarged long arm. An explanation of the intermediary position of the women in regard to hereditary factor VIII deficiency is given in terms of the lyonization theory, according to which there is in female cells a genetically inactive X-chromosome.