The transfusion requirements of 75 adolescents with severe haemophilia A were studied during the five-year period 1973-77. The annual incidence of the 4935 episodes studied increased by a factor of 2.2 while the number of transfusions rose by a factor of 2.5 and the amount of therapeutic material used during the five years of the survey increased by a factor of 2.6. A further 166 bleeds occurred during periods of prophylaxis in 1976 and 1977, which generated a 25% increase in factor VIII used during those years. The increased usage of factor VIII in the years 1976 and 1977 was thus due mainly to increased numbers of transfusions given per bleed and to the use of prophylaxis but also to a slight increase in the units of factor VIII given in each dose. Twice weekly prophylaxis reduced the bleeding frequency by 30% and resulted in an increase of about 12% in usage of factor VIII. Prophylaxis given three times weekly reduced the bleeding frequency by about 60% at the cost of an increase of 77% in therapeutic materials. Iliopsoas, retroperitoneal, and abdominal wall bleeds were the commonest bleeds needing retransfusion within 24 hours, while bleeds into the extremities of the upper and lower limbs needed least retransfusions within 24 hours. Retroperitoneal bleeds needed the most transfusions per episode followed by iliopsoas, buttock, abdominal wall, and hip joint bleeds. The transfusion requirements of bleeds below the diaphragm tended to diminish steadily the more peripheral they became. This relationship did not hold for upper limb bleeds.