Agenesis of internal carotid artery (ICA) is rare. Association of agenesis with abnormal arterial communication between the cavernous segment of the carotid arteries is extremely rare. We present a case of a middle-aged woman who presented with sudden onset severe holocranial headache. She was investigated and diagnosed to have sub-arachnoid hemorrhage on CT scan. Digital Subtraction Angiogram revealed absent right internal carotid artery. The right middle cerebral artery received blood supply from an incidentally detected intercavernous anastomosis from the left internal carotid artery. No aneurysm, arteriovenous malformation or any other vascular abnormality was detected. The patient was managed conservatively. She has been asymptomatic for five years on follow up.