Optokinetic nystagmus was tested in patients with intracerebral tumors restricted to one hemisphere, by using a simple, hand-held drum and visual evaluation of symmetry of horizontal response. Computed tomographic mapping revealed that lesions associated with asymmetric optokinetic nystagmus involved the posterior hemisphere. The critical region appeared to extend from the posterior splenium and the anteromedial occipital lobe to the posterior internal capsule. It seemed to course below the posterior horn of the ventricular system, then turn anteriorly, and finally medially, above the temporal horn. It is argued that the posterior extremity of this C-shaped region contains the ipsilateral and contralateral visual inputs to a parietal cortex control over ipsilateral smooth eye movements, and that the anterior extremity contains the efferent motor command pathways destined to the brain stem.