A 49 year old female presented with subarachnoid hemorrhage due to a ruptured dissecting aneurysm on the left vertebral artery (VA). Following an occlusion test, we performed proximal occlusion of the left VA with detachable balloons. However, a dissecting aneurysm on the right VA developed three weeks later. After an occlusion test had showed no change in cerebral blood flow, auditory brain stem response, or neurological status, proximal occlusion of the right VA was performed. The patient has returned to normal life without neurological deficits. Bilateral dissecting aneurysms of the VA are quite common, but de novo VA dissecting aneurysms or enlargement of such aneurysms after occlusion of contralateral VA are rare. This case suggests that hemodynamics stress may be a causal factor in the development of VA dissecting aneurysms. Careful pre- and post-operative neuroradiological examination of the contralateral VA are required in patients undergoing VA occlusion for dissecting aneurysms.