Ventricular sizes on computerized tomographic (CT) scans were compared in seven patients with Huntington disease, 20 patients with cerebral atrophy, and 20 normal controls. The bicaudate index--the ratio of the width of both lateral ventricles at the level of the heads of the caudate nuclei to the distance between the outer tables of the skull at the same level--significantly discriminated among the three groups. The bicaudate indices were: Huntington disease 0.209 +/- 0.007, cerebral atrophy 0.121 +/- 0.006, and controls 0.092 +/- 0.003.