A case of a primary non-Hodgkin's lymphoma of the CNS is reported in which a complete and stable disappearance of an enhancing mass lesion on the CT scan after treatment with dexamethasone was followed by the occurrence of a high-grade malignant lymphoma in the opposite hemisphere 18 months later. The long-lasting and stable remission suggests a direct oncolytic effect of corticosteroids on lymphoma cells. Furthermore, this case illustrates the usefulness of repeated CT examinations in patients with CNS lymphoma and steroid treatment.