We examined ten patients from three families with von Hippel-Lindau disease and 26 of their at-risk relatives for the presence of twin vessels, defined as a paired retinal arteriole and venule that are separated by less than the diameter of one venule and extend for a distance of more than one disk diameter. They were compared with 36 age- and sex-matched controls. Of the 36 subjects in the study group, 23 had twin vessels compared with two controls (P less than 10(-6). Of the ten patients, nine (14 eyes) had twin vessels; no twin vessels were found in their controls (P = 5.9 x 10(-5)). Fourteen at-risk relatives and two of their controls had twin vessels (P = 9.4 x 10(-4)).