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Acquired hemophilia A (AHA) is a rare bleeding disorder caused by an inhibitor to endogenous factor VIII. The pathogenic mechanism is considered to be the breakdown of the immune system associated with several conditions such as malignancy, autoimmune diseases, and pregnancy. We report a case of AHA that was diagnosed based on a hematoma of the oral floor during follow-up for prostate cancer with medicationrelated osteonecrosis of the jaw (MRONJ) . An 81-year-old man visited our hospital with the chief complaint of bone exposure following extraction of the upper right canine and the first and second premolars. He was diagnosed with MRONJ due to his course involving denosumab treatment for prostate cancer bone metastases. During conservative treatment, dark red swelling of the oral floor and subcutaneous hemorrhaging in his extremities were observed three months after the first visit. Blood test results indicated an endogenous coagulation system abnormality, and he was referred to the department of hematology and diagnosed with AHA. Hemostasis was achieved within a few days using bypassing agents and immune tolerance induction.Factor VIII antibodies disappeared once but were detected again. Therefore, immune tolerance induction is ongoing.