All known surviving haemophiliacs A and B and their relatives were reexa-mined by laboratory and clinical methods and evaluated by a genetic-epidemiologic approach in 4 north-western counties of Hungary. The prevalence of haemophilia A and B patients born in the fifties was 2.73 and 0.25 per 10,000 persons, respectively. The reproductive fitness was found to be 0.3 in haemophilia A, and 0.8 in haemophilia B patients. The mutation rates calculated by the indirect method were 6.3 × 10-5 for haemophilia A and 0.2 × 10–5 for haemophili