A 50-year-old man had a primary lymphoma of the central nervous system one year after a tick bite and a radiculoneuritis secondary to Borrelia burgdorferi infection. Determination of antibodies against Borrelia burgdorferi revealed increasing IgM and IgG titers, and the autopsy showed a primary B-cell immunoblastic lymphoma of the brain without evidence of extraneural lymphoproliferative disorder. Lymphoma of the brain is a rare type of central nervous system cancer, and sporadic cases appear without predisposing features such as immunosuppression or viral infection. The controversy surrounding the histogenesis of this neoplasm is reviewed: an unknown agent or a cofactor should provoke cellular proliferation and the formation of a lymphoma.