Introduction: Limited data are available on optimal prophylaxis regimens of factor IX (FIX) replacements for patients with haemophilia B. Aim: This multicentre, open-label study evaluated the efficacy and safety of once-weekly prophylaxis with nonacog alfa compared with on-demand treatment in adolescent and adult patients. Methods: Males aged 12-65 years with moderately severe to severe haemophilia B (FIX:C <= 2%) were eligible for enrolment. Patients received on-demand treatment for 26 weeks, followed by once-weekly prophylaxis of 100 IU kg(-1) for 52 weeks. The primary efficacy end point was the annualized bleeding rate (ABR). Secondary end points included response to on-demand treatment, the number of infusions used to treat bleeding events, and the incidence of less-than-expected therapeutic effect (LETE). FIX: C was measured on day 1 and at weeks 26 and 78. Results: Mean (+/- SD) ABR was lower during prophylaxis vs. on-demand treatment [3.6 (+/- 4.6) vs. 32.9 (+/- 17.4) events, respectively; P < 0.0001]. The majority (88.4%) of bleeding events had excellent or good responses upon the first infusion; 82.1% of events responded to the first infusion. No incident of LETE occurred. No thrombotic events or FIX inhibitors were reported. Eight of 17 FIX: C approximately 1 week after dosing were >2 IU dL(-1) (min-max of 2.13-10.39 IU dL(-1)). Conclusions: Once-weekly prophylaxis of 100 IU kg(-1) was associated with lower ABR compared with on-demand treatment in adolescents and adults with moderately severe to severe haemophilia B. Once-weekly prophylaxis was well tolerated, with a similar safety profile as that reported during the on-demand treatment period. Residual FIX: C may be supportive of effectiveness.