We report a case of a 58-year-old female with neuromeningeal relapse of a large B-cell lymphoma which developed a meningoencephalitis due to adenovirus (ADV). Diagnosis of suspected viral meningoencephalitis was based on computed tomography (CT) scan radiological pictures, which were atypical for CNS neoplastic infiltrations, and it was confirmed by direct immunofluorescence performed on cerebrospinal fluid. Cultural exams identified adenovirus type 7. The ADV infection resolved after treatment with total four doses of cidofovir. Unfortunately, the hematological disease progressed and the patient died 2 months later from disseminated lymphoma.