Colloid cysts of the third ventricle are described in middle-aged twin brothers. One of them presented with recurrent attacks of headache. In this patient the cyst had reached a size large enough to obstruct the cerebrospinal fluid pathway, resulting in hydrocephalus. The twin brother, although asymptomatic, was suspected of the anomaly and investigated because of the similarity of his ocular signs. The diagnosis was confirmed by computerized tomography in both the patient and his brother. The latter proved to have a smaller colloid cyst situated anteriorly in the third ventricle with no obstructive hydrocephalus. The patient was successfully operated on, while the brother is still under observation. Both brothers have had bilateral cataracts, retinal detachments, and left lateral rectus palsies. The familial occurrence of colloid cysts and their association with these ocular findings have apparently not been described before.