In order to obtain complete success in the carrier and prenatal diagnoses required for genetic counselling in haemophilia B a new strategy is being implemented in the UK. This entails the construction of a national confidential database of mutations, pedigrees and haematological data. This will allow the inefficient indirect tests based on the analysis of DNA polymorphisms to be abandoned and direct detection of the gene defect to be used instead. After two and a half years of nationwide collaboration, 702 samples have been collected from 313 families, representing more than half of the UK haemophilia B families, and 217 mutations have been characterised. The 141 diagnostic tests so far performed have clearly indicated that the new strategy not only allows virtually 100% diagnostic success, but also rapid results. This work on haemophilia B may represent a model for other diseases with high mutational heterogeneity.