We present a case of a ruptured aneurysm of the medial trunk of the right posterior inferior cerebellar artery(PICA) in the fourth ventricle. A 52-year-old man presented with sudden onset of headache, followed by an abrupt decline in consciousness. Angiographic studies revealed that the aneurysm arose from the medial trunk of the right PICA, distal to the choroidal point and associated with the stenosis of the basilar artery and the hypoplasia of the right anterior inferior cerebellar artery and superior cerebellar artery. The patient underwent a midline suboccipital craniotomy and aneurysmectomy after clipping of the parent artery. A postoperative angiogram confirmed complete obliteration of the aneurysm, but demonstrated occlusion of the vermian branch of the right PICA. The pathogenesis of this lesion could be due to hemodynamic stress.