OCTANINE F is a high-purity blood clotting factor IX concentrate that has been shown to be effective and safe in adults with haemophilia B. At present, there are no prospective clinical study data on FIX replacement therapy in young children. The primary objective of this trial was to investigate the immunogenicity of OCTANINE F in children aged <6 years with haemophilia B. Secondary objectives were to assess the efficacy, viral safety and tolerability of OCTANINE F in this patient population. Twenty-five children aged <6 years with moderate or severe haemophilia B, including six who were previously untreated and 13 who had less than previous 50 exposure days were assigned to prophylactic or on-demand treatment with OCTANINE F over a 12- to 24-month period. Immunogenicity was assessed at baseline, during the treatment period and at the end of treatment by monitoring the levels of inhibitor. OCTANINE F was not associated with the development of an inhibitor in any patient during the study; all patients had a FIX inhibitor level of <0.4 Bethesda units (BU) for all samples taken throughout the study. The efficacy of OCTANINE F was rated as excellent in 96.4% of 499 bleeding episodes and tolerability was rated as very good in 97% of 1684 injections. OCTANINE F was shown to be effective and well tolerated in children aged <6 years with moderate or severe haemophilia B, including previously untreated patients, with no reported cases of FIX inhibitors or thrombotic events.