A 42-year-old woman presented with very rare cases of ruptured saccular aneurysm of a dolichoectatic internal carotid artery (ICA) associated with agenesis of the contralateral ICA manifesting as sudden onset of severe headache and nausea without neurological deficits. Angiography and three-dimensional computed tomography demonstrated intraventricular hemorrhage with slight subarachnoid hemorrhage and dolichoectasia of the right ICA with agenesis of the contralateral ICA, as well as a saccular aneurysm of the ectatic right ICA. The aneurysm neck was clipped successfully. The patient remained ambulatory with no neurological deficits at discharge 15 days after the surgery. The saccular aneurysm in our case was formed in the dolichoectatic ICA, presumably due to both abnormal hemodynamics and abnormal arterial wall.