Hemophilia B Leyden is characterized by low levels of factor IX antigen and activity before the age of 15, whereas after puberty factor IX levels rise at a rate of about 5% per year. A single base substitution (-A----T) at position -20 was identified in the putative promoter of the gene cloned from a patient with hemophilia B Leyden. This nucleotide change was confirmed in a second patient from the same pedigree and was also found in a patient from a second Dutch pedigree with the same hemophilic phenotype. The results indicate that the two Dutch kindreds are related and point to the functional significance of the -20 position for the expression of the human factor IX gene.