Optokinetic nystagmus (OKN) at a constant stimulus velocity of 90 degrees/s was studied in 50 healthy subjects and three different groups of patients: 10 with vestibular neuritis (VN), 17 with unilateral infratentorial lesion (UI) and 13 patients with bilateral infratentorial lesion (BI). Among healthy subjects, OKN decreased with age in those over 42 and 44 years for maximum and mean velocity, respectively. The maximum velocity of OKN discriminated between the healthy subjects and the groups of patients rather better than did mean velocity of OKN. Asymmetry of either mean or maximum velocity contributed only marginally to correct group assignment. By subjecting test data on velocity and asymmetry variables to linear discriminant function analysis, 80% of subjects could be correctly classified as healthy/not healthy, and 64% assigned to the correct group according to diagnosis. Analysis of OKN data satisfactorily discriminated BI and UI patients from VN patients and healthy subjects, and is thus useful in assessing infratentorial vestibular lesions.