We investigated the presence of twin vessels in two patients and in four of their relatives at risk from one family with autosomal-dominant hereditary cavernous hemangioma of the retina associated with central nervous system involvement. Twin vessels were detected in four of the six patients examined. The proband had bilateral retinal vascular hamartomas with central nervous system involvement but no twin vessels. The proband's mother had vascular hamartomas of the retina and brain with twin vessels. In the other three family members, twin vessels were associated either with retinal cavernous hemangiomas (one patient) or with normal fundi (two patients). Because twin vessels may be an ocular manifestation of von Hippel-Lindau disease, their presence in one of our two patients and in the otherwise healthy three family members suggests that twin vessels may be associated with different retinal vascular hamartomas, including capillary and cavernous hemangiomas.