An intracranial aneurysm in one identical twin, but no aneurysm in the other.

Only seven pairs of identical twins with intracranial aneurysms have been reported in the literature. In six of these, both twins had aneurysms, but in one pair only one twin had an aneurysm. We present another such pair of identical twins 21 years of age of whom only one twin had an aneurysm. She presented with a subarachnoid haemorrhage. Four-vessel angiography showed one aneurysm on the internal carotid artery; the aneurysm was clipped. Four-vessel angiography in her identical twin sister revealed no aneurysms. We have reviewed the seven previous reports of identical twins with intracranial aneurysms. We conclude that in case of an aneurysm in one identical twin, the other twin should be examined.