Background:Spontaneous complete thrombosis of a giant aneurysm and its parent artery is a rare event. Their spontaneous recanalization is even rarer, with few reports.Case Description:A 17-year-old male patient presenting blurred vision and headache, with a history of seizures, was referred to our service. After further investigation with cranial computed tomography, magnetic resonance imaging (MRI), and cerebral angiography (CAG), it was diagnosed a thrombosed aneurysm of the posterior cerebral artery (PCA) and also complete thrombosis of the PCA. Three years later, he experienced visual worsening. A new MRI scan indicated flow both through the aneurysm and the left PCA, which was further confirmed by CAG. We decided for a noninterventional treatment combined with strict clinical follow-up. The patient continues to present with the previous neurological deficit, without recurrence of headaches.Conclusions:Thrombosis is not the final event in the natural history of giant aneurysms, and partial thrombosis does not preclude the risk of rupture. Thrombosed aneurysms may display additional growth brought about by wall dissections or intramural hemorrhages. Their treatment may be either surgical or involve endovascular procedures such as embolization. Thrombosed giant aneurysms are dynamic and unstable lesions. A noninterventional treatment is feasible, but aneurysmal growth or recanalization may suggest the need for a more active intervention.