Hemophilia A is a congenital bleeding disorder caused by deficiency of factor VIII (FVIII) and is treated by replacement therapy with FVIII concentrate. Approximately 30% of people with severe hemophilia A develop neutralizing antibodies, called FVIII inhibitors, which interfere with the function of FVIII concentrates. The reason that some, but not all, people with severe hemophilia A develop inhibitors is incompletely understood. Understanding individual and environmental risk factors is important to be able to prevent and possibly treat inhibitors. This study will look at individual and treatment characteristics in babies with severe hemophilia A who have not yet received treatment with FVIII (called Previously Untreated Patients, or PUPS). Subjects in the study will be asked to provide diaries of treatments, medications, and illnesses. Treatment will be directed by the subjects' physician, but all subjects will receive Advate, a third-generation recombinant FVIII product. Subjects will have blood drawn for laboratory tests, which include studies of the immune system and genetic studies of the FVIII mutation, before and 7-9 days after the first treatment with FVIII, and 5 days (+/-2 days) after the 5th, 10th, 20th, 30th, 40th, and 50th days of treatment with FVIII (exposure days). The duration of the study will be first 50 treatments or 3 years, whichever comes first.

Inclusion Criteria:

• Severe hemophilia A with FVIII activity < 1% normal
• Weight > 3.5 kg at the time of baseline study evaluation
• Informed consent, approved by appropriate Institutional Review Board/Independent Ethics Committee, has been administered, signed, and dated

Exclusion Criteria:

• Prior exposure to clotting factor concentrates or blood products
• Other chronic disease
• Currently participating in another investigational drug study.

Patients with severe hemophilia A who have not previously been treated with Factor VIII concentrates.