Cerebral lymphomas are one of the complications of AIDS. Their increasing frequency, especially in the last decade, has to be correlated with the incidence of systemic lymphomas and especially central nervous system (CNS) lymphomas in non immuno-compromised patients. CNS involvement is found in 40% of systemic lymphomas. Primary lymphomas have an incidence of 0.4-0.56% in epidemiological studies and 7.6% in post-mortem series. Primary lymphomas are noted in all patients, homosexuals, drug-addicts, children, with a lower rate in the haemophiliacs. Known risk factors are HIV itself Epstein-Barr virus almost constantly demonstrated in tumour cells. Epidemiological studies on cerebral lymphomas must still be completed as, until recently, they were included in the group entitled lymphomas, and risk factors have to be specified with more accuracy.