Two types of lymphoproliferative disease associated with Epstein-Barr viral (EBV) DNA--central-nervous-system lymphoma and chronic lymphocytic interstitial pneumonitis (LIP)--were recognised in children with human T-lymphotropic virus type III/lymphadenopathy virus infection and the acquired immunodeficiency syndrome (AIDS). Eight of ten lung biopsy specimens from children with LIP contained EBV DNA. EBV DNA was not identified in lung biopsy specimens with Pneumocystis carinii, cytomegalovirus, or atypical mycobacteria but without LIP, nor was EBV found in lung biopsy specimens from five adults with AIDS. Children with these EBV-associated complications had raised serum antibody titres to the replicative EBV antigens, but most of them lacked antibody to the component of the EB nuclear antigen encoded by the Bam HI K fragment. EBV-associated lymphoproliferative disease is a common and important complication of childhood AIDS.