Certain mutations in mild haemophilia A have been associated with a greater risk of inhibitor development, especially when associated with intense treatment. We present a patient with both mild haemophilia A and Arg531Cys mutation, which developed lowtitre inhibitors and was not seen to be related to the intense substitute treatment. The inhibitor has a greater effect on the exogenous factor VIII, permiting an adequate response to treatment with desmopressin. A discrepancy exists in the factor VIII activity in this our patient and in the haemophiliacs of another two families with the same mutation when determination is performed with one-stage or chromogenic method.