INFECTIOUS mononucleosis is generally regarded as a benign disease having a uniformly good prognosis. In the majority of cases this belief is substantiated, but in occasional cases this disease causes permanent damage and sometimes seriously threatens life.The clinical features of infectious mononucleosis have been described in detail123456 and there are numerous articles on the hematological findings.1,2,5,78910 There have, however, been relatively few reports dealing with the abnormalities of the central nervous system sometimes encountered.The first case of involvement of the central nervous system in infectious mononucleosis was reported by Johansen11 in 1931. His patient presented meningismus, drowsiness, impaired . . .