A computer program was developed for on-line analysis of nystagmus parameters using a microcomputer (SORD M243EX). In this paper, the clinical utilities of microcomputer analysis of optokinetic nystagmus (OKN) were described, and quantitative analysis of each parameter of OKN was performed in 20 normal subjects and 49 patients with central nervous system (CNS) disorders. The average gain was accurately measured and the maximal stimulus velocity during which time the gain was more than 0.8, was termed as the "optokinetic adaptation limit (OAL)," and used as a clinical index for evaluation of OKN abnormalities. All patients in the present study showed a remarkable decrease in OAL. The differences in OAL among these patients at each level of lesion site, however, proved to be statistically insignificant. The relationship between the direction of OKN limitation and the lesion side was investigated. In all patients who had lesions on the thalamus, midbrain, and cerebellum, OKN limitation was observed toward the lesion side. The OKN limitation, however, was seen toward the intact side in 7 of 11 patients with medullary lesions and in 2 of 11 patients with cerebellopontine angle (CPA) tumors. hence, OKN in a useful diagnostic tool for the assessment of infratentorial lesions and computer analysis of OKN may be clinically useful.