In this clinical report, we present the management of a brain abscess that was presumed to be caused by radiation osteomyelitis of the mandible. The patient underwent chemoradiotherapy for oropharyngeal carcinoma on the left side at the Department of Otolaryngology of our hospital in 2000. Local recurrence or metastasis did not occur post-treatment. In January 2009, the patient was diagnosed with radiation osteomyelitis of the mandible on the left side, complicated by a pathologic fracture. In July 2011, numbness occurred in the left upper extremity and the patient was transferred to the emergency center of our hospital. A computed tomography scan showed a tumor lesion in the brain that was diagnosed as a brain abscess by magnetic resonance imaging. Neurosurgeons performed burr hole drainage of the abscess, followed by administration of antibiotics. Although impaired fine movements of the left hand remain, progress has been relatively favorable with no recurrence of osteomyelitis or brain abscess post-surgery.