A 63-year-old man developed bilateral paresis of horizontal and upward eye movements. He was found to have a small oat cell carcinoma of the lung. Four months later he experienced acute visual blurring on the right side. Examination of the right eye at that time revealed a visual acuity of 3/200 and a central scotoma. There was swelling of the right optic disc. Three weeks after the onset of the visual loss, the acuity of the right eye spontaneously improved to 20/60, the field deficit lessened, and there was a decrease in the swelling of the optic disc. Subsequently, his neuro-ophthalmologic condition remained unchanged but his general health deteriorated, and he died nine months after the onset of the disease. Neuropathologic examination showed mild perivascular lymphocytic infiltration and fibrosis of the meninges throughout the central nervous system, loss of neurons and gliosis in the third and fourth cranial nerve nuclei, perivascular inflammation and gliosis of the optic nerves, and chiasm and central demyelination of the right optic nerve. No tumor cells were seen. These findings were consistent with a diagnosis of paraneoplastic optic neuritis and paraneoplastic encephalomyelitis. The present case confirms the existence of paraneoplastic optic neuritis and illustrates the clinical course of the disease.