Type and quantity of replacement treatment, together with haematological and immunological parameters were determined in 37 boys with severe haemophilia A and 41 children with other bleeding disorders. The quantity of factor VIII concentrate given to boys with severe haemophilia A (mean U/year) showed a significant inverse correlation with total white cell counts, lymphocyte counts, platelet counts, and the ratio of monoclonal antibody defined T lymphocyte subsets, T4 and T8 (T4:T8). Of the boys with severe haemophilia A, 49% had inversed T4:T8 ratios and 24% had thrombocytopenia. Treatment with high dose factor VIII concentrate (more than 25 000 U/year) was associated with low platelet counts, low lymphocyte counts, low T4:T8 ratios, and hypergammaglobulinaemia. In addition, six patients with severe haemophilia A and factor VIII inhibitors had inversed T4:T8 ratios. Patients treated exclusively with cryoprecipitate or prothrombin complex concentrates had normal T4:T8 ratios and platelet counts. The severity of the haematological and immunological abnormalities observed seems to be associated with high usage of factor VIII concentrates. Similar abnormalities have been described in patients with the acquired immune deficiency syndrome (AIDS). Prospective study of haemophiliacs is required to assess long term sequelae of factor concentrate usage, including the possible development of AIDS.