The authors report a rare case of eosinophilic cystitis presenting with heavy haematuria which led to the discovery of attenuated haemophilia B. Bleeding and clotting times in this patient, with no past history of haemorrhage, were normal. Complete coagulation studies led to the diagnosis in the presence of poor plasma prothrombin consumption and a moderate deficit in clotting factor IX, defining haemophilia B. Despite two partial cystectomies for haemorrhagic lesions of the dome of the bladder (eosinophilic cystitis), carried out with transfusion cover and the administration of P.P.S.B., recurrence of haematuria made it necessary to "rest" the bladder by a cutaneous ureterostomy. Subsequently, the restoration of urinary continuity was possible by uretero-ileo-cystoplasty, without any haemorrhagic complications by virtue of the use of P.P.S.B. The value of the association of epsilon amino-caproic acid with P.P.S.B. is emphasised in the prevention of haemorrhagic complications during surgery in haemokphilia B patients. In attenuated forms of haemophilia B (factor IX level between 5 and 15%) haemorrhages may be only occasional and occur only late in life.