We report the second confirmed case of the haemophilia B 'Brandenberg' phenotype. At the time of testing, patient HB530 was a 17-year-old post-puberty male with a persistent, clinically severe bleeding disorder and markedly reduced plasma procoagulant factor IX activity (< 1%). Sequencing studies revealed a G-->A transition at bp - 26 within the promoter region of the factor IX gene. This case report confirms the observation that not all patients with promoter mutations improve after puberty and supports the hypothesis that bp - 26 is a critical binding site within the factor IX gene promoter region for both constitutive as well as androgen-inducible transcription factors.