Plasma from 15 patients with haemophilia A and 4 patients with von Willebrand's disease (VWD) has been investigated for the presence of antibodies to von Willebrand factor (VWF) using ELISA and immunoblotting systems. All plasma samples were also analyzed for anticoagulant activity against factor VIII coagulant activity (FVIII:C) and ristocetin co-factor activity of VWF (RCof). The patients were on regular prophylaxis or treated 'on demand' with different commercial FVIII-VWF complex concentrates. Neutralizing activity against FVIII:C was only found in haemophiliacs (in 3 out of 15), whereas anti-RC of was only found in VWD patients (in 2 out of 4). In ELISA, antibodies to VWF were detected in 3 of the VWD patients. However, using the immunoblotting system, antibodies to the multimeric structure of VWF were found not only in VWD patients (in 3 out of 4); remarkably 9 out of 15 haemophiliacs also reacted with VWF, including some that did not show FVIII:C inhibitors. The pattern of VWF multimers obtained with the antibodies showed differences among the patients. These findings are discussed together with the mechanisms by which such antibodies could have developed in haemophilia A patients who are not deficient in this particular protein (VWF).