Of the 45 haemophilia-B patients registered at the haemophilia centre in Malm?, Sweden, 24 are the sole members of their families to be affected, and in 13 of these 24 cases, ascendant relatives are available for study. Detection of the gene defect showed the mutation to be de novo in the proband in 3 of these 13 cases, and inherited from a carrier mother in the remaining 10 cases. All 10 carrier mothers were shown to have de novo mutations, as the patients' grandfathers were phenotypically and/or haematologically normal, and the grandmothers were non-carriers. Seven restriction fragment length polymorphisms (RFLPs) of the factor IX gene were used to determine whether the de novo mutations of the 10 carrier mothers were of paternal or maternal origin. In 6/10 cases, the RFLP patterns were informative, and indicated the mutation to be of paternal origin.