Primary lung cancer with acquired hemophilia A is rare. We report a case of large cell neuroendocrine carcinoma (LCNEC) with acquired hemophilia A diagnosed by endobronchial biopsy. A 75-year-old male was admitted to our hospital due to severe anemia and multiple organ failure. Hematomas, prolonged activated partial thromboplastin time, decreased activity of factor VIII, and the presence of serum factor VIII inhibitors led to the diagnosis of acquired hemophilia A. Administration of recombinant activated factor VIII and steroid treatment resolved the coagulation abnormalities. Simultaneously, a CT scan showed a lung mass and an endobronchial biopsy confirmed the diagnosis of LCNEC. Although acquired hemophilia A can cause severe bleeding symptoms, a lung cancer diagnosis by bronchoscopy was possible under appropriate treatment of acquired hemophilia A.