The horizontal optokinetic nystagmus (NOC) has been studied during 5,000 otoneurological investigations. In 16.25 % of all our cases, the NOC was found to be either weakened in both directions, or only in one direction. There has been no constant relationship between intensity of the spontaneous nystagmus and NOC asymmetry in case of peripheral vestibular lesion. NOC asymmetry seems to be a reliable sign of the evolution of a lesion. Cases without peripheral hypo function have been divided into three groups: ocular disturbances, brain stem lesions and hemispherical lesions. In these cases the value of the localization of the asymmetry in NOC and nystagmus induced by vestibular tests has been studied. In hemispherical lesions, when asymmetry affected exclusively the NOC, the direction of the weakened NOC was always towards the healthy side. When asymmetry affected not only NOC but even nystagmus induced by vestibular stimulations, this weakened side of the nystagmus was less significant.