Involvement of the central nervous system in infectious mononucleosis is rare, but probably not as rare as the dozen or so cases thus far reported would seem to indicate. What is more likely is that the nervous system involvement so masks the other manifestations of the disease that the diagnosis is overlooked.1The nervous symptoms may appear as much as two weeks before the characteristic lymphadenopathy and splenic enlargement2resulting, if the characteristic blood picture is overlooked, in the erroneous diagnosis of lymphocytic choriomeningitis, acute toxic encephalitis, epidemic encephalitis or early poliomyelitis. The neurologic symptoms produced may be multiple. All patients have headaches. There is a varying degree of dulling of the sensorium from mild lethargy to coma. The patient may be irritable and irrational.1There may be photophobia, nystagmus and poor articulation.3Peripheral neurologic symptoms, such as facial nerve palsy, oculomotor nerve involvement, optic neuritis