BackgroundHereditary factor X (FX) deficiency (FXD) affects 1:500 000‐1:1 000 000 people worldwide. A novel, high‐purity plasma‐derived FX concentrate (pdFX) is available in the United States and European Union as replacement therapy for FXD, but data are scarce on pdFX use in children <12 years.AimThis prospective, open‐label phase 3 study assessed the safety, efficacy and pharmacokinetics of pdFX in children <12 years with moderate/severe FXD.MethodsSubjects aged <12 years with basal plasma FX activity (FX:C) <5 IU/dL received pdFX as prophylactic and on‐demand treatment, with doses adjusted to maintain FX:C > 5 IU/dL. After ≥26 weeks and ≥50 exposure days, investigators rated pdFX efficacy for preventing/decreasing bleeds. Secondary endpoints included number and severity of bleeds, trough FX:C and incremental recovery. Safety parameters were adverse events (AEs), inhibitor development and changes in laboratory parameters.ResultsThe study enrolled 9 subjects (0‐5 years, n = 4; 6‐11 years, n = 5) with severe (n = 8) or moderate (n = 1) FXD. At end of study, investigators rated pdFX efficacy excellent for all subjects. Ten bleeds occurred (n = 3 subjects; 6 major, 3 minor, 1 unassessed for severity). Trough FX:C levels remained >5 IU/dL for all subjects after the last dose adjustment study visit. Mean incremental recovery was significantly lower for younger vs older subjects (1.53 vs 1.91 IU/dL per IU/kg; P = .001). All AEs were unrelated to treatment; no inhibitor development or clinically significant changes in laboratory parameters were observed.ConclusionsThese results demonstrate the efficacy and safety of pdFX for treating children <12 years with moderate/severe hereditary FXD.