The study aimed at characterizing the putative changes in the epitope specificity of anti-FVIII antibodies during a successful immune tolerance treatment of the haemophilia A patient with the factor VIII (FVIII) preparation containing the von Willebrand factor (VWF). At the beginning of treatment, anti-FVIII inhibitory antibodies recognizing predominantly the light chain of FVIII were prevalent and persisted throughout the treatment. More detailed characterization of the FVIII antibody epitope specificity by using GST-fusion proteins corresponding to different FVIII domains revealed the prevalence of C1-domain-specific antibodies, while a remarkably lower amount of antibodies were targeted at the C2 and the a3 domains of the FVIII light chain and towards the A2 and the A1 domain of the FVIII heavy chain. The epitope specificity of antibodies remained rather unchanged throughout treatment except the elevated level of C2-domain-specific FVIII antibodies after a temporary interruption of treatment. The patient's antibodies were unable to interfere with the FVIII binding to VWF or to phospholipids, but inhibited FXa generation and the binding of FX to FVIII on the phospholipid monolayer. Thus, a unique pattern of the epitope specificity of FVIII antibodies and the mechanism to inhibit FVIII:C activity by FVIII-light-chain-specific antibodies were characterized.