Prophylactic administration of factor VIII to prevent joint damage in boys with haemophilia A has had a chequered history. This prophylaxis became popular in the 1960s but was dropped in the 1980s because of the transmission of human immunodeficiency and hepatitis viruses. A recombinant factor VIII became available in the early 1990s, eliminating the risk of such infections. It was shown that prophylaxis could slow down the progression of established joint disease, but it has not been proved that it prevents joint disease. Now a multicentre US trial (Marilyn J Manco-Johnson and colleagues. New England Journal of …