Optokinetic nystagmus (OKN) was studied in ten patients with vestibular neuritis, and in seventeen patients with unilateral and thirteen patients with bilateral infratentorial lesions and compared with OKN in fifty healthy subjects. Mean and maximum slow phase velocity of OKN was calculated as well as the asymmetry of responses. The efficiency of the different variables in OKN, in discriminating between different lesion sites, was tested in a linear discriminant function analysis. Slow phase velocity of OKN could satisfactorily separate patients with infratentorial lesions from healthy subjects or subjects with vestibular neuritis. Asymmetry of OKN did not contribute further to the correct diagnosis.