Isolated reduced coagulation activity of FVIII may be a manifestation of haemophilia A, carriership of haemophilia A, haemophilia A in a woman, acquired haemophilia A and type 2N of von Willebrand's disease. The authors were concerned with the cause of isolated reduction of the coagulation activity of factor VIII (19 IU/dl) in a 40-year-old woman with a history of excessive haemorrhage of the type of mild haemophilia A with a negative family history. The personal history, family history and laboratory examination suggested type (variant) 2N of von Willebrand's disease. For indirect evidence the authors used a therapeutic study where they investigated the effect of administration of a concentrate of coagulation factors VIII/von Willebrand's factor (1/2), 28 IU factor VIII/kg body weight, on the coagulation activity of factor VIII. They recorded a half-life prolonged to 53 hours as compared with controls where the half-life was less than 12 hours. The therapeutic study confirmed sufficient coagulation activity of factor VIII, the utilization of which improved as a result of administration of von Willebrand's factor. This investigation confirmed indirectly as the cause of reduced coagulation activity of factor VIII in the examined patient the assumed type (variant) 2N of von Willebrand's disease.