Congenital factor XI deficiency (also known as the Rosenthal syndrome or 
hemophilia C) manifests as minor bleeding, usually after trauma or surgery. We 
report a case in which bilateral knee hemarthrosis was the first manifestation. 
The patient presented at 32 years of age with a 2-year history of mechanical 
pain and intermittent swelling in both knees. Knee aspiration recovered 
blood-tinged fluid. The laboratory workup showed severe factor XI deficiency. 
Replacement therapy with fresh frozen plasma was effective. Tests in the family 
showed factor XI deficiency in the patient's sister.