With the advent of antibiotics, severe complications of acute otitis media have decreased significantly. Bezold's abscess, which is a well known complication of acute otitis media, is an extremely rare disease in Japan.We report a case of masked mastoiditis which later went on to Bezold's abscess. The patient was a 17-year-old male who was admitted to our department with complaints of pain and swelling of the right postauricular and right neck regions.Physical examination on admission revealed intact tympanic membranes but there was slight tenderness in the right postauricular region. X-ray films showed radiopacity in the right mastoid region, and CT scan revealed a soft tissue density in the right mastoid air cells and abscess formation deep to the sternocleidomastoid muscle.The patient was operated on under general anesthesia. The right neck was explored through an anterior skin incision, and an abscess was observed beneath the right sternocleidomastoid muscle. It was drained.A radical mastoidectomy on the right ear revealed granulation tissue in the mastoid cavity and a defect in the mastoid cortex. No defect was observed in the facial canal or semicircular canals.The postoperative course was uneventful. The patient was discharged 30 days after the operation.Acute otitis media is cured easily with antibiotics. However, when antibiotics therapy is incomplete, acute otitis media develops without evident symptoms and severe complications sometimes occur. We stress in this paper the careful use of antibiotics in treating acute otitis media.