A case of dural arteriovenous malformation (AVM) in the posterior cranial fossa detected after STA-MCA anastomosis surgery. A 52-year-old male consulted a neighbourhood hospital for sudden headache and vomiting. He was diagnosed as having intraventricular hemorrhage on CT scan. Though the obstruction of the right internal carotid artery was revealed angiographically, his symptoms improved after conservative therapy. Two weeks after onset, his consciousness deteriorated and he developed left hemiparesis. Thereafter, he was transferred to our hospital. After thorough examination, right STA-MCA anastomosis surgery was performed. Approximately 2 months after surgery, right tinnitus developed and gradually exacerbated. Since it was thought to be due to increased blood flow in the right superficial temporal artery, it was kept under observation. On angiogram, 8 months after surgery, good blood flow supplied from the right superficial temporal artery to the territory of the right middle cerebral artery was shown, and a dural AVM fed by the right occipital artery was found. Fourteen months after the surgery, an enlarged dural AVM with backflow to the superficial cerebral veins fed by the enlarged right occipital artery and right ascending pharyngeal artery was revealed. Embolization therapy to the right occipital and ascending pharyngeal artery was performed using coils and ivalon, and irradiation of 30 Gy was added. After this treatment, right tinnitus improved. On angiography 2 years later, transverse sinus was slightly visible via the right occipital artery and ascending pharyngeal artery, but the dural AVM was significantly reduced. The origin of dural AVMs remains controversial. In our case, dural AVM was not found before the STA-MCA anastomosis surgery, and sinus thrombosis was not found throughout the course of observation. It is thought that the occult dural AVM was disclosed and enlarged by the increased blood flow through the external carotid artery via the STA-MCA anastomosis. Therefore, the dural AVM seemed to be congenital in origin.