We report the case of a 66-year-old woman with a subarachnoid hemorrhage (SAH) caused by a de novo vertebral artery-posterior inferior cerebellar artery (VA-PICA) aneurysm. The aneurysm had developed within a time span of only 2 years and 7 months. Although she suffered from SAH due to a ruptured left internal carotid artery aneurysm and had received a 4-vessel-angiography 2 years and 7 months before, there was no VA-PICA aneurysm present in the angiogram. In general, hemodynamic changes, hypertension, and congenital anomalies seem to be major risk factors for the formation of an aneurysm. However, this patient did not display such risk factors. In this report, we also discuss the clinical features of de novo aneurysms that are not accompanied by the above-mentioned risk factors. We emphasize the importance of a follow-up angiography. Most reported cases of de novo aneurysms seem to be identified after SAH due to these types of aneurysms. At present, we are able to use a three-dimentional CT angiography (3D-CTA) and magnetic resonance angiography (MRA) more easily and less invasively than a conventional angiography. Therefore, frequent follow-up studies using 3D-CTA and/or MRA are recommended for patients with a ruptured aneurysm.