29 boys (4-18 years old) with severe haemophilia A were given prophylactic infusions of AHF concentrate (human fraction I-0) for 2 to 13 years in an attempt to change the haemophilia from a severe to a moderate form and thereby prevent arthropathy and severe bleeding episodes. The sizes of the doses and the intervals at which the doses were given were titrated by AHF survival studies. As a rule, the patients received AHF in amounts sufficient to raise the AHF level to 30-45% at 5-12 day intervals. In about 50% of the infusions the AHF content was not below 1% before the next infusion. During such prophylaxis all patients except one have been in a good general condition. They have had bleeding episodes, which have, however, been much less severe and less frequent. The children have been able to live an almost normal life. The number and duration of stays in hospital have been markedly reduced. 17 of the patients had only minor or no joint defects before the start of the treatment. In this group the joint function was identical with that found in moderate haemophilia in the same age groups. Two patients developed anticoagulants. No other side effects were seen. The prophylactic regimen in Sweden thus reduced severe haemophilia to moderate.