There is increasing evidence that congenital thrombotic thrombocytopenic purpura (TTP) is caused by an absolute deficiency of von Willebrand factor-cleaving protease. The recent identification of this protease and the development of assays for its detection have enabled its quantification in a number of plasma products, including some commercial intermediate-purity plasma-derived factor VIII preparations. We report the successful, weekly prophylactic use of a commercial intermediate-purity plasma-derived factor VIII concentrate in the treatment of a 14-year-old girl with severe congenital TTP who had previously required transfusions of fresh-frozen plasma every 2 weeks from the age of 4 months.