This paper describes a national surveillance scheme, began in 1995, designed to monitor immunosuppression associated with HIV infection in adults in England and Wales. Currently 60 out of 64 (94%) laboratories performing CD4 cell counts participate in the scheme. The database contains over 42,000 patient records with over 300,000 counts, taken between April 1984 and March 2000. Approximately half of the patient records in the CD4 database match with patient records in the UK database of diagnosed HIV infections; a large proportion of the unmatched patients in the CD4 database may not be HIV-infected. Close to 50% of both men who have sex with men and injecting drug users and two-thirds of those who acquired their infection heterosexually had CD4 cell counts below 350 cells/mm3 at the time of HIV diagnosis. The National CD4 Surveillance Scheme provides important information regarding the epidemiology of HIV infection such as the changes in patterns of early and late diagnosis. It should continue to be used in conjunction with the other HIV surveillance systems to present as complete a picture of the epidemic as possible.