Electro-oculograms (EOGs) were recorded from 14 patients with a malignant melanoma of the choroid, nine of which were histologically verified. Twelve of the 14 eyes (86%) with a malignant melanoma of the choroid had an EOG value that was considered borderline. One eye studied had a normal EOG value and a histologically verified malignant melanoma. EOGs were also recorded from a group of eight patients with a choroidal nervus. Five of these eight eyes had normal EOG values, two eyes had borderline EOG values, and one eye had an abnormal EOG value. The EOG value in eyes with a malignant melanoma of the choroid was inversely related to the size of the melanoma. Abnormal EOG values were obtained, however, from eyes lesions occupying only a few square millimeters.