Introduction Infection with HIV causes a spectrum of clinical problems beginning at the time of seroconversion (primary HIV) and terminating with AIDS and death. It is now recognised that it may take 10 years or more for AIDS to develop after seroconversion. The Centers for Disease Control (CDC) in the USA developed the most widely used classification for HIV disease based on the presence of clinical symptoms and signs, the presence of certain conditions and investigative findings, the availability of HIV screening and the degree of immunosuppression as measured by the CD4 lymphocyte count. The infection is divided into four groups: This article has been adapted from the forthcoming 5th edition of ABC of AIDS. The book will be available from the BMJ bookshop and at www.bmjbooks.com Summary of CDC 1992 classification system for HIV disease Group I Primary HIV Group II Asymptomatic infection Group III Persistent generalised lymphadenopathy Group IV Symptomatic infection Group IVA HIV wasting syndrome (AIDS) and constitutional disease Group IVB HIV encephalopathy (AIDS) and neurological disease Group IVC1 Major opportunistic infections specified as AIDS defining Group IVC2 Minor opportunistic infections Group IVD Cancers specified as AIDS defining Group IVE Other conditions Group I Primary HIV infection Group II Asymptomatic phase Group III Persistent generalised lymphadenopathy Group IV Symptomatic infection Group IV is subdivided into several subgroups and some of these (groups IVA, B, C1 and D) are AIDS defining conditions. In 1992 the CDC included all HIV infected persons with CD4 lymphocyte counts of <200×106 cells/l as fulfilling an AIDS defining diagnosis. However, this additional classification is not widely used outside the USA. A second classification also combines clinical and CD4 count information. Symptoms and clinical findings are graded in severity from A to C and CD4 counts as they fall from 1 to 3.