True congenital peripheral aneurysms of the cerebral arteries are rare and may constitute a special group. The authors report on 16 cases of peripheral aneurysms of the cerebellar arteries; six of them arising from vessels feeding arteriovenous malformations (AVM's). The possible causative role of increased blood flow versus developmental factors in the frequent occurrence in cases of AVM of this otherwise rare type of aneurysm is discussed. In five of the six cases with AVM's (one dural and five parenchymal), the aneurysm rather than the AVM proved to be the source of the hemorrhage. The occurrence of spasm was in inverse proportion to the distance of the aneurysm from the main arteries. The surgical result was good in 11 cases. One patient with associated AVM died, and four patients had residual symptoms.