We describe the case of a 82 years old female, who has been referred to the emergency department for a grosstrauma of the right knee after an accidental fall. Physical examination revealed a palpable mass in the right pelvis,which was then identified as a large intramuscular hematoma of the right iliacus muscle by computerized tomography scan. The most suggestive laboratory findings were anemia and a prolonged activated partial thromboplastin time (APTT) (ratio 1.33), with physiological platelet count and prothrombin time. After ten days of hospitalization, when a spontaneous hematoma developed in the right arm, APTT had steadily increased, up to a value of 3.33. A mixing study and assessment of coagulation factors were rapidly performed. The former test was not effective to normalized the APTT, whereas concentrations of all factors were within the reference interval, except for factor VIII (0.6%). Factor VIII inhibitor titration using Bethesda assay confirmed the diagnosis of acquired hemophilia A, yielding a value of 77 Bethesda units. Acquired hemophilia A, which is caused by autoantibodies against coagulation factor VIII, is a rare condition that can be frequently overlooked or misdiagnosed. The role of laboratory diagnostics is thereby as important as the clinics, wherein serious hemorrhages accompanied by variable APTT prolongations onset in a previously asymptomatic patient. Along with discussion about laboratory and clinical aspects of acquired hemophilia.