A dural arteriovenous fistula (AVF) of the posterior fossa was found in a 58-year-old businessman suffering from a month's history of dementia. Bilateral transverse sinus thrombosis was also found and his symptom was attributed to venous congestion caused by the sinus occlusion. He had received partial resection of the right trigeminal neurinoma two years before, and the operation was thought to have caused the sinus occlusion. Dural AVF of the posterior fossa usually causes tinnitus or headache, whereas dementia seen in present case is relatively rare. In the present case, serial study of cranial CT scan demonstrated evolution of dural AVF after sinus occlusion. Venous congestion was thought to re-open the physiological micro-AVF to cause the dural AVF.