Discrete lesions interrupting the fiber tracts that connect the pontine center for conjugate horizontal gaze and the ipsilateral abducens nucleus create the syndrome of Lutz's posterior internuclear ophthalmoplegia. In this syndrome the lateral rectus on the side of the lesion fails to abduct the eye normally on attempted horizontal gaze to the same side. Adduction of the contralateral eye is normal. This syndrome may be differentiated from the more common abducens nerve palsy by noting the orthotropic position of the eyes in primary position, the absence of primary and secondary deviations with alternate fixation, and the absence of diplopia. A case of this syndrome, presumably due to central nervous system vasculitis, is reported, and related human and animal studies are reviewed.