Both optokinetic nystagmus (OKN) and fixation-suppression of caloric nystagmus have been proposed as useful diagnostic tools for assessment of infratentorial lesions. However, despite all efforts to establish diagnostic values in the brainstem lesions over the years. Localizing values ofderanged OKN and failure of fixation-suppression against a discrete lesion site have not been established yet. In this study, both slow phase OKN velocity and fixation-suppression of caloric nystagmus were investigated on 44 patients, who showed discrete unilateral lesions in the brainstem and in the cerebellar hemisphere.1. Both OKN and fixation-suppression of caloric nystagmus were impaired toward the side of the lesions in all patients who had lesions in the midbrain.2. OKN were limited toward the side of the lesions, whereas fixation-suppression were equivocal data between normal and abnormal in all patients who had the lesions in the ventral tegmentum of the pons.3. Both OKN and fixation-suppression were impaired toward the contralateral side to the lesions in patients who had lesions in the dorsal tegmentum of the pons.4. OKN were limited toward the side of the lesions, whereas fixation-suppression were lost toward the contralateral side to the lesions in all patients who had lesions either in the superior or the middle cerebellar peduncle.5. In 4 of 8 patients with Wallenberg' s syndrome, both OKN and fixation-suppession were impaired toward the side of the lesions. In the other 4 patients with Wallenberg's syndrome, OKN were limited toward the contralateral side to the lesion, whereas fixation-suppression were impaired toward the lesion sides.6. Both OKN and fixation-suppression were impaired bilaterally, in the patients who had cerebellar lesions.It seems that these findings are useful for the diagnosis of localization of the infratentorial lesions.