Hereditary haemorrhagic telangiectasia (HHT, Osler-Weber-Rendu syndrome) is a rare inherited disease; acquired haemophilia, caused by factor VIII inhibitors, is a rare autoimmune disorder. Both diseases cause bleeding manifestations. We report a case of a young woman affected by hereditary haemorrhagic telangiectasia who was admitted to our unit because of gross hematuria. The prolonged activated partial thromboplastin time suggested us the possibility of a bleeding cause other than hereditary haemorrhagic telangiectasia. The test result for factor VIII inhibitors resulted positive and a diagnosis of acquired haemophilia was made. The patient was treated with frozen fresh plasma and with activated eptacog alpha and a steroid therapy was started. After she received steroid, factor VIII inhibitor titre decreased and activated partial thromboplastin time gradually recovered. After 2 weeks a complete normalization of activated partial thromboplastin time was obtained. Acquired haemophilia is a life-threatening disorder. Measuring factor VIII activity and its inhibitor in patients with bleeding and a prolonged activated partial thromboplastin time are recommended even in patients affected by other known hemorrhagic disease.