We describe the unusual association of two ocular motor abnormalities: upbeat nystagmus and internuclear ophthalmoplegia in a young woman originally believed to have demyelinating disease. Quantitative eye movement recordings documented a unidirectional defect in upward visual pursuit with preservation of the vertical vestibulo-ocular reflexes. The ocular oscillation was characterized as a type of "pursuit defect" nystagmus. An enhancing lesion of the upper brainstem seen on computerized tomography was primarily localized to the midbrain on pneumoencephalography. Biosy of the lesion disclosed a malignant glial tumor. The patient's course initally progressed but became stable following radiotherapy and chemotherapy during the next eight months. We believe the association of upbeat nystagmus and internuclear ophthalmoglegia should prompt a detailed search for a structural lesion.