Primary non-Hodgkin lymphoma of the brain is rare, representing only 1% of all non-Hodgkin lymphomas (NHLs), but its incidence has been increasing rapidly in south-east England since 1985. Among 17,322 cases of NHL registered during the 18 year period 1973-90, there were 210 cases of primary cerebral NHL, of which 179 (86%) were diagnosed in the last third of this period, 1985-90. This increase in cerebral lymphoma is not adequately explained by improvements in the precision of diagnosis or by changes in disease coding or cancer registration practice. While there has also been a rapid increase in Kaposi sarcoma, neither immunosuppression acquired through HIV infection nor the overall trend in non-Hodgkin lymphoma can satisfactorily explain the recent increase in cerebral lymphoma, which affects all ages and both sexes similarly. Other possible causes for a true increase in cerebral lymphoma should be sought.