OBJECTIVE To report long-term imaging follow-up and clinical outcome of 13 patients with a giant vertebrobasilar aneurysm treated by parent artery occlusion (PAO). METHODS From 1994 to 2000, 13 consecutive patients with a giant vertebrobasilar aneurysm were treated by PAO. Symptoms were related to mass effect in nine patients and to a subarachnoid hemorrhage in four. Endovascular treatment consisted of aneurysm trapping in nine patients and occlusion of one or both vertebral arteries in four. We assessed the clinical outcome and imaging findings in all patients during a 28-month period. RESULTS Endovascular treatment resulted in clinical improvements in eight patients, worsening of symptoms in four, and death in one. One woman with a ruptured vertebral aneurysm died from a rebleeding after PAO without trapping. One man developed a brainstem infarction after lower basilar artery occlusion and incurred hemiparesis. In three patients, symptoms of mass effect increased after the procedure. Long-term follow-up revealed good or excellent clinical outcome in all patients and a sharp decrease in size of the thrombosed aneurysm in nine patients. One basilar aneurysm recanalized despite selective coiling and subsequent bilateral vertebral artery occlusion; one vertebral aneurysm and one basilar aneurysm did not decrease in size despite complete occlusion. CONCLUSIONS Giant vertebrobasilar aneurysms are rare and challenging lesions for both neurosurgeons and neurointerventionalists. Their treatment by endovascular PAO remains safe and effective. Early clinical worsening may be observed, but long-term follow-up shows good or excellent results in most patients. This treatment can be carried out with minimal morbidity and mortality using clinical and angiographic monitoring.