This is a case report of a patient with unruptured dissecting aneurysm in the vertebral artery that bled after being treated by proximal clipping. A 53-year-old male was admitted to our hospital due to transient right hemiparesis which occurred 20 days prior to his admission. He had been medicated for hypertension for the previous 33 years. CT scan and MRI showed lacunar infarction in the left corona radiata, and an aneurysm was accompanied with clot in the prepontine cistern. Angiography revealed a dissecting aneurysm in the right intracranial vertebral artery. His right hemiparesis was derived from infarction in the left corona radiata. It was likely that the dissecting aneurysm might rupture in the future. Proximal clip ping was performed to prevent rupture of the aneurysm. After clipping of the right vertebral artery distal to the PICA, the wall of the aneurysm appeared to be drawn toward the clip blades and to be tensed by the blades. Four hours after the operation, he complained of severe headache, and experienced a sudden loss of consciousness and the immediate development of a deep comatose state. CT scan disclosed massive SAH in the right cerebellopontine and basal cistern. Repeat angiography demonstrated that the aneurysm was not visualized and the right vertebral artery distal to the aneurysms was opacified through the left vertebral artery. Ventricular drainage was performed, but the patient died on the 20th day after bleeding. It was suspected that the aneurysmal clip might have produced shear force on the weak adventitia of the dissecting aneurysms and that the intra-aneurysmal pressure might have increased because of blood back-flow via the contralateral vertebral artery after the proximal clipping.