The joint status (knees, ankles) of patients suffering from severe Hemophilia A (too little blood clotting factor VIII in blood) is evaluated in a single magnetic resonance imaging session. No study medication is given.

Inclusion Criteria:

• Males aged 12 - 35 years
• Severe hemophilia A ( < 1 % FVIII:C)
• No history of Factor VIII inhibitory antibody
• For prophylaxis groups, having received at least two prophylactic infusions per week for 45 weeks per year for the prevention of bleeding without relevant interruption and continuing until the present.
• Complete documentation of joints bleeds and their locations prior to start of prophylaxis,
• Bleeding history and/or treatments received during the last 5 years documented in the subjects medical records.
• For the on-demand subjects > 12 bleeds/year in the last 5 years.
• Written informed consent by subject and parent/legal representative, if < 18 years

Exclusion Criteria:

• Individuals with other coagulopathies (e.g., von Willebrand disease)
• HIV seropositive subjects
• Individuals for whom the most clinically severe joint is not one of the 4 index joints (ankle, knee)
• HCV seropositive individuals who underwent interferon therapy during the last 12 months
• Individuals for whom high-magnetic exposure is contraindicated (see section 7.1)
• Synovectomy performed within the six months prior to investigation enrollment or orthopedic surgery planned to be performed within the investigation period
• Joint replacement
• For the on-demand treatment group, any period greater than 8 consecutive months having received >/= 50 IU per kg per week Factor VIII for the prevention of bleeding