Seventy-nine out of 172 patients with abnormal horizontal or vertical optokinetic nystagmus had a specific condition or disease that was diagnosed following a complete ENG. Ocular vertigo, MS, motion sickness, brain stem lesions, cerebellar tumors, cerebellar atrophy, parietal lobe tumors, and Harada syndrome were the diagnoses made. Optokinetic asymmetry--horizontal, vertical, or a combination of these--was the most consistent finding in these conditions (Table). In six patients with cerebellar tumor, abnormal findings on OPK testing suggested the need for further evaluation when physical and neurologic examinations, brain scans, and EEG were normal. Surgery confirmed the diagnosis of cerebellar tumor with these patients. This study suggested the importance of using five progressively faster drum speeds in both the horizontal and vertical planes. All patients who had proven or strongly suspected MS had eye-speed fatigability as the drum speed increased, that is, as the drum speed increased, the eye speed decreased. It is suggested that optokinetic studies be included in the ENG. Vertical asymmetry, although not as common as horizontal, should be included in that 14 patients had only this finding.