Study design. The study consisted of a 1-month screening period, 14-day PK assessment, followed by active treatment of at least 12 months. Patients were assigned to either prophylaxis treatment (group 1) or on-demand treatment (group 2) based on previous treatment regimen. Group 1 patients received weekly prophylaxis for 26 weeks and then were evaluated for eligibility to switch to a longer treatment interval (star). Group 2 received on-demand treatment of the first 26 weeks followed by weekly rIX-FP prophylaxis for an additional 26 weeks or longer. Subjects were required to have no spontaneous bleeds for at least 1 month and be on a stable dose of ≤40 IU/kg or ≤50 IU/kg rIX-FP to switch to a 14-day or 10-day treatment interval, respectively. Subjects continued on 7-, 10-, or 14-day prophylaxis for the remaining treatment period. A subset of subjects in group 1 (the first 15 patients naïve to rIX-FP to enroll) repeated the PK assessment of 50 IU/kg rIX-FP at approximately week 26, prior to switching to a longer treatment interval. EOS, end of study.