This study will provide an opportunity for systematic observation of treatment with rFIX in children less than 6 years of age regardless of prior FIX treatment. Younger patients exhibit a different pharmacokinetic profile and therefore may respond differently to rFIX infusions when compared with older children and adults. This evaluation will provide data from which recommendations can be made regarding rFIX dosing and treatment of these patients. Surveillance for certain observations that have been made in patients treated with rFIX in the clinical and postmarketing setting will be performed, including inhibitor development, thrombogenicity, FIX recovery/lack of effect, allergic-type manifestations, and RBC agglutination. Comparisons will be derived from published reports and communications describing experience with other FIX products and protein therapeutics in general.