We report a Lancashire family with mild haemophilia B associated with a -6 G to A mutation in the factor IX promoter. This mutation has been reported previously in Britain in one other affected family. The factor IX haplotype of these two unrelated patients was determined by PCR analysis of seven polymorphic sites within or close to the factor IX gene. This analysis involved the intragenic Msp I polymorphism which we have now located precisely to allow the design of suitable PCR primers. The haplotype of both of these patients was identical suggesting but not proving a possible common origin for the mutation.