Primary CNS lymphomas are extranodal malignant lymphomas arising in the CNS in the absence of obvious lymphoma outside the nervous system at the time of diagnosis. They should be differentiated from secondary involvement of the nervous system in systemic lymphomas. The incidence of primary CNS lymphomas has recently increased markedly worldwide from about 1% to over 6% of primary intracranial neoplasms due mainly as a consequence of the AIDS epidemic. Primary CNS lymphomas affect all ages, with a peak incidence in immunocompetent subjects during the sixth and seventh decade of life and a male:female ratio of 3:2. In immunocompromised patients, the age at manifestation is lowest in individuals who have an inherited immunodeficiency (10 years), followed by transplant recipients (37 years) and AIDS patients (39 years, 90% males). About 60% of primary CNS lymphomas involve the supratentorial space, including the frontal (15%), temporal (8%), parietal (7%) and occipital (3%) lobes, basal ganglia/periventricular regions (10%) and corpus callosum (5%), the posterior fossa (13%), and the spinal cord (1%). The majority of primary CNS lymphomas are large B-cell lymphomas, often associated with the Epstein-Barr virus. (Adapted from WHO.)