A 19-year-old immune-competent patient developed right-sided headache and, subsequently, subacute diplopia. On clinical examination he had incomplete right oculomotor palsy. Cranial MRI showed pathologic contrast enhancement of the right oculomotor nerve at its exit point from the mesencephalon, and the CSF displayed slight pleocytosis. The following relevant differential diagnoses were not supported by additional examinations: neurosarcoidosis, Lyme neuroborreliosis, neurosyphilis, tuberculous meningitis, viral meningitis (HIV, VZV, CMV), CNS lymphoma, vasculitis associated with rheumatic disease, Tolosa-Hunt syndrome, and diabetic neuropathy. However, on the basis of blood lymphocytosis, positive heterophile antibody test (Paul-Bunnell test), the presence of IgM antibodies against Epstein-Barr virus capsid antigen, and elevated transaminases, infectious mononucleosis was diagnosed. Isolated neuritis of the oculomotor nerve is a rare parainfectious manifestation of infectious mononucleosis.