Acquired haemophilia A is a rare autoimmune disorder, in which antibodies are formed against coagulation factor VIII. The effect of the inhibitor on blood clotting results in severe, life-threatening bleeding diathesis. The patient's life depends on the rapid diagnosis: besides the characteristic clinical presentation, a prolonged activated partial thromboplastin time, which is not corrigible with normal plasma, suggests the existence of the disorder. In the case of the female patient who was referred to our hospital due to severe anaemia, acquired haemophilia A was diagnosed rapidly, the day after her admission. We used activated prothrombin complex concentrate to stop the bleeding, and introduced immunosuppressive therapy. The early recognition of the disease and immediate initiation of adequate treatment resulted in the patient's full recovery. With our case presentation, we would like to draw attention to the fact that the rapid diagnosis of acquired haemophilia A depends on the determination of a simple, easily accessible coagulation parameter, the activated partial thromboplastin time and on the immediate recognition of its incorrigible prolongation.