Background <p>The association between basilar tip aneurysms and bilateral internal carotid artery (ICA) hypoplasia is an exceptionally rare clinical entity that poses unique diagnostic and therapeutic challenges. Fewer than ten cases have been documented in the global literature.</p> Case Presentation <p>A 42-year-old woman with a history of subarachnoid hemorrhage previously treated by endovascular coil embolization developed a subacute, progressive brainstem syndrome characterized by right faciobrachial–crural hemiparesis, dysarthria, dysphagia, and a disabling rest-and-postural head tremor of rubral phenotype. Vascular imaging identified a recurrent giant basilar tip aneurysm (~ 50&#xa0;mm) and bilateral ICA hypoplasia, with the entire anterior circulation supplied by enlarged posterior communicating arteries. The aneurysm produced significant pontomesencephalic compression and posterior circulation hemodynamic stress, accounting for the patient’s progressive neurological decline. The case was discussed in a multidisciplinary neurovascular setting to guide individualized management.</p> Conclusion <p>This is one of fewer than ten cases worldwide of this rare association. The combination of bilateral ICA hypoplasia and a giant basilar aneurysm creates unique hemodynamic conditions that promote aneurysm formation and complicate therapeutic decision-making, underscoring the importance of long-term surveillance and individualized treatment strategies.</p>