THE OCCURRENCE of nervous system involvement as a complication of infectious mononucleosis has been recognized with increasing frequency in recent years. The first report of such an association was by Johansen1in 1931; later in the same year Epstein and Dameshek2reported a case in this country. In 1950, Bernstein and Wolff,3reviewing the literature from 1931 to 1947, found 28 reports, involving a total of 46 patients, with neurologic complications of infectious mononucleosis. They estimated that the incidence of this complication is very low, "certainly not over one per cent, and probably less." This estimate appears justified, as Contratto,4in a series of 196 cases, and Milne,5with 141 cases, observed no instance of nervous system involvement. Despite its low incidence, nervous system involvement ranks high among the causes of death in infectious mononucleosis. Lawrence6notes that of 16 reported fatalities7resulted from neurologic complications. Spontaneous rupture of the