This is a multi-center cohort study of approximately 250 previously untreated patients (PUPs) with congenital moderate to severe hemophilia A or B in a network of up to 50 US Hemophilia Treatment Centers (HTCs). Participants will be followed as they receive their first 50 exposure days (ED) to clotting factor replacement product, both prospectively and retrospectively. The data collected on evolving treatment practices will define the incidence and risk factors for inhibitor development during the high risk period of first 50 ED and improve the outcomes of this vulnerable population.

This multi-center, longitudinal, observational, prospective and retrospective study of previously untreated patients (PUPs) with moderate to severe hemophilia A or B during the initial 50 exposure days (ED) to clotting factor replacement product or until the development of a confirmed inhibitor. The Primary Investigators have designed the study to utilize the American Thrombosis and Hemostasis Network (ATHN) electronic infrastructure to leverage existing data and enable the collection of more in-depth clinical and laboratory data on PUPs. The study aligns with the National Hemophilia Foundation Medical and Scientific Advisory Council (MASAC) recently issued Recommendation #243 which includes: "Regardless of which option is chosen, all PUPs should be enrolled in the ATHN data collection system or a clinical trial to assess outcomes." Co-enrollment in the ATHNdataset by participants is required. The total study duration is planned for 6 years.

The primary objective is to determine the percentage of patients with confirmed inhibitors within the first 50 ED. Confirmed inhibitors are defined as two consecutive positive inhibitor titers (per CDC laboratory criteria; >0.5 Nijmegen Bethesda Units for hemophilia A and >0.3 Nijmegen Bethesda Units for hemophilia B) on different blood samples which result in change in treatment recommendations.

Please note - the treatment regimen will be at the discretion of the participants' hemophilia caregivers. No treatment products are being provided by the study nor will the participants be paid. However, inhibitor titer testing will be provided at no cost to participants by the Centers for Disease Control and Prevention (CDC).

All study procedures and follow-up will be timed to coincide with scheduled hemophilia care whenever possible. Ad hoc, quarterly follow-up, annual and final visits are for participants who have not met study endpoints (50 ED or inhibitor development) prior to enrollment.

Data collected will include eligibility, demographics, medical history (co-morbidities, surgery/procedures, immunizations and allergies), hemophilia history (severity, genotype and family history), birth history, inhibitor testing results, detailed treatment product(s) usage, bleeding events, bleeding disorder related medical visits during the study, and EUHASS adverse events.

Sub-studies

A number of sub-studies are planned with pharmaceutical sponsors to collect information from patients about their specific product use. Participation in these product specific sub-studies is optional and sub-study visits will be planned to coincide with HTC visits. The sub-study will collect information from patients about their perception and use of treatment products, physical activity levels and other general health questions. This data will be collected via questionnaire.

Data Collection System

All data collected will be entered into electronic case report forms (eCRFs) within the secure ATHN System by HTC site personnel. All participating study sites will have in place a current, executed Data Use and Business Associate Agreement (DUBAA) with ATHN.

Inclusion Criteria:

• Congenital hemophilia A; FVIII \

The study will enroll a cohort of 250 participants born with hemophilia on or after January 1, 2010 who meet the eligibility criteria and are receiving care from one of the participating HTCs.