The objective of this paper was to report a rare complication of basilar artery (BA) tourniquet treatment of a giant basilar tip aneurysm, and to discuss possible causes for the formation of a de novo giant posterior cerebral artery (PCA) aneurysm. A 34-year-old woman underwent satisfactory treatment of a ruptured giant basilar bifurcation aneurysm by BA ligation (Drake tourniquet) in 1985. She presented 25 years later with a new aneurysm in the left PCA, successfully treated by coil embolization. To the authors’ knowledge, this is the first case of de novo aneurysm formation on a PCA, and the first de novo aneurysm reported as a complication of BA ligation therapy by Drake tourniquet. Long-term follow-up is necessary in patients with treated cerebral aneurysms, particularly those occurring in young patients, those with multiple aneurysms, those with complex posterior circulation aneurysms, and those undergoing flow diversion or flow-altering therapies.