The authors report three different and rare forms of neurological complication associated with infectious mononucleosis. The first two, which are faily unusual in their clinical signs, occurred during the course of two typical cases of mononucleosis, one in the form of progressively regressive myelitis and the other in that of amyotrophic paralysis of the shoulder. The third occurred in isolation, after an encephalitic attack combined with a state of epileptic mal, with no other accompanying infectious sign. On the basis of these three single cases, and recent data in the literature about the Epstein-Barr virus, the authors briefly recapitulate the neurological clinical forms of infectious monouncleosis and the means of serological diagnosis; they discuss the frequency and pathogeny of these complications and tentatively suggest certain similarities with other nervous disorders observed in the course of proliferative or dysglobulinaemic diseases.