A 58-year-old woman complaining of a mild headache was admitted to our hospital. MRI 3 months before admission revealed a round lesion at the right quadrigeminal cistern. Cerebral angiograms demonstrated a fusiform aneurysm arising from the parietooccipital artery, which is the distal branch of the right posterior cerebral artery. Repeated MRI and cerebral angiograms performed on admission demonstrated complete thrombosis of an aneurysm and the parent artery without any clinical symptoms. This is the first case of complete spontaneous thrombosis of an aneurysm of the distal posterior cerebral artery. The mechanism of its development and spontaneous thrombosis in a fusiform aneurysm is discussed.