A factor VIII inhibitor has been found in a patient with an unusual combination of factor VIII-related properties. The inhibitor is directed specifically against the clotting activity (VIII:C) of the factor VIII complex. It behaves in a similar fashion to high responding inhibitors of factor VIII seen in haemophilia A patients and it was characterised as an immunoglobulin of the IgG class. Laboratory results from the patient and his family show considerable variation of factor VIII-related properties between various individuals. Overall, the data suggests the co-existence of haemophilia A and von Willebrand's disease in the family and the presence of both diseases in the patient.