A case of intraventricular arachnoid cyst in an elderly man is reported. A 63-year-old man developed a progressive gait disturbance over a five-year period. CT scan showed a large cyst in the left lateral ventricle which was negative to contrast enhancement. CT cisternography revealed gradual accumulation and more than 48 hours retention of contrast medium in the cyst. The patient underwent left frontal craniotomy, and the cyst wall was partially resected for histopathological examination. Although limit microscopic examination could not establish a diagnosis, arachnoid cyst was diagnosed by electron microscopic findings. Biochemical analysis did not detect any difference between cyst fluid and CSF obtained during surgery. It is suggested that a ball-valve mechanism caused progressive enlargement of the cyst and gradual development of symptoms in this elderly patient.