An extremely rare case of ruptured cerebral aneurysm associated with a fenestrated vertebral artery in osteogenesis imperfecta (OI) is presented. A 33-year-old female suffering from OI was admitted to our hospital with severe headache and vomiting. A CT scan revealed subarachnoid hemorrhage. Cerebral angiography with four vessel study showed a fenestration in the V3 portion of the left vertebral artery and a dilatation in its V4 portion, but no cerebral aneurysm was detected. After conservative treatment for three weeks, repeated angiography demonstrated an aneurysm of the anterior communicating artery. A neck clipping of the aneurysm was performed successfully. The patient was discharged with no neurological deficits. OI is a hereditary connective tissue disease characterized by bone fragility. From her family history and clinical findings, the patient was suspected to have OI type I of Sillence's classification. Among the connective tissue diseases, OI does not have complications in the cerebrovascular system as frequently as other connective tissue diseases do, for example, Marfan's syndrome, Ehlers-Danlos syndrome, or pseudoxanthoma elasticum. Carotid-cavernous fistula and moyamoya disease have been the only complications reported in OI. However, dilatation of the aortic root and increased vascular fragility have been reported recently in OI. Although this is the first reported case of a ruptured aneurysm accompanied by a fenestration and a dilatation of the vertebral artery associated with OI, it was suggested that vascular fragility caused by collagen abnormality might affect the cerebral vasculature.