Hemophilia type A or B is due to deficiency in factor VIII C or IX C, but whatever the type and whether the affection is severe or attenuated the risk of hemorrhage after surgery is identical. Willebrand's disease is due to either a qualitative or quantitative anomaly of willebrand's factor. Between 1979 and 1984, 15 children with these diseases were operated upon for removal of tonsils and adenoids. Surgery was performed under cover of substituted therapy with frozen cryoprecipitate for Willebrand's disease, either frozen or dried cryoprecipitate together with F VIII concentrates for hemophilia A, and PPSB for hemophilia B. This treatment was continued pastoperatively for 7 days after adenoidectomy and 10 days after tonsillectomy. Follow up examinations enabling possible adjustment of transfusional needs included determination of CKT and assay of factors VIII C or IX C in the hemophiliac children, and assay of factors F VIII R, AG, F VIII RCF and F VIII C in those will willebrand's disease.