Primary lymphomas of the central nervous system (CNS) are rare diseases. Often these tumors are surrounded by glia cells and may, therefore, be misdiagnosed as 'astrocytomas' with accompanying reactive lymphocytosis. A 15-year old patient was irradiated to the posterior cranial fossa and the brain stem because of a supposed astrocytoma. Five months after completion of radiotherapy he presented two lesions each in the right and left cerebral hemisphere. Repeated biopsy led to a revision of the primary diagnosis in favor of a B-cell Non-Hodgkin lymphoma (centroblastic type). After cyclic polychemotherapy including high-dose methotrexate and cytosine-arabinoside he entered a complete remission. No further radiotherapy was given. So far, 18 months after discontinuating therapy, the patient has been in complete remission and is in an excellent physical condition.