We describe two rare cases of de novo basilar head aneurysms. The first patient presented with a ruptured basilar tip aneurysm, which developed de novo 5 years after the clipping of a left middle cerebral artery aneurysm. The second patient presented with a right basilar artery-superior cerebellar artery aneurysm, which developed de novo 3 years after the clipping of a right A₁ aneurysm. The formation of de novo aneurysm in the posterior circulation has significant implications for screening. Patients must be carefully examined not only at the sites of the initial aneurysm, but also at separate sites, including the posterior circulation, to evaluate the possibility of de novo appearance of another aneurysm.