Study Evaluating BeneFIX in Patients With Haemophilia B, Previously Treated With Plasma Derived Factor IX

Inclusion Criteria: * Moderately to severe haemophilia B patient (FIX activity \< or equal to 2%) for whom the switch from pdFIX to BeneFIX has already been decided by the investigator * Previously treated patients (PTP) with \> or equal to 150 ED to any FIX product * Male patients, aged \> or equal to 12 years * Absolute CD4 count \> or equal to 300/microL * Normal platelet count (\> or equal to 100 000/microL) * Patient is in a non-bleeding state and has not received any coagulation FIX within five (5) days of recovery * Written informed consent obtained prior to study entry (for patients aged \< 18 years, parents' signature or subject legally acceptable representative obtained prior to study entry) Exclusion Criteria * Any other known bleeding disorder in addition to haemophilia B * History of, or current detectable factor IX inhibitor (\> or equal to 0.6 BU by Bethesda inhibitor assay) * History of anaphylaxis to any coagulation factor IX * Patient with a known hypersensitivity to hamster protein * Patient with a hypersensitivity to the active substance or to any of the excipients * Patient unable to be off FIX replacement therapy for at least 5 days without bleeding Patient with hepatic or renal impairment (ALT \[SGPT\] and AST \[SGOT\] \> 5 x Upper Limit Normal (ULN), total bilirubin \> 20mg/l, albumin \< 25 g/l, prothrombin time \> 1.25 x ULN, serum creatinine \> 1.25 x ULN) * Treatment with any investigational drug or device within the past 30 days * Any condition that, in the Investigator's judgment, makes participation in the study not advisable