The patient was first admitted on August 19, 1975 with 5 years' headache and tinnitus. Physical examination revealed bruit over the left mastoid area. Left angiography, performed in August, 1975, revealed a dural arteriovenous malformation, which was supplied by enlarged left middle meningeal artery, occipital artery, meningohypophyseal artery and superior cerebellar artery and was draining into the left sigmoid sinus. At the first operation, the left external carotid artery was ligated at the neck, and then coagulation of dural vessels was performed through left temporo-occipital craniotomy. Tinnitus and bruit were disappeared after operation. Postoperative right angiogram showed no residual dural AVM, however, ligation of the right external carotid artery was required to reduce symptomes. In June, 1979 the left brachial arteriography was performed, and revealed a dural AVM around the left sigmoid sinus supplied by anastomotic vessels between external carotid artery and cervical artery arising from the vertebral artery. Because of subarachnoid hemorrhage, repeated angiography in April, 1982, revealed progressively enlarging dural AVM, although no aneurysm was seen in the left internal carotid arterial system. The patient readmitted on October, 28, 1983 with severe headache and confusion. CT scan showed slightly high density in the left basal cistern. Angiogram showed no dural AVM and a saccular aneurysm arising from the left internal carotid artery at C2 portion, which was successfully clipped by surgery. Only two cases of spontaneous regression of dural AVM in the posterior fossa were reported previously, and the authors added a case, which showed a newly developed aneurysm after spontaneous regression of the posterior fossa dural AVM.