The purpose of this study is to collect data around the period of the conversion from plasma-derived Factor IX (pdFIX) to BeneFIX. The main information collected will be: a retrospective history of the bleedings in the 3-month period before the conversion, the recovery with pdFIX just before the conversion and with BeneFIX just after the conversion, and a prospective history of the bleedings in the 3 month period following the conversion.

The switch to BeneFIX has already been decided by the investigator. Patients will be followed up to 3 months after the switch.

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