PIP: HIV and AIDS case surveillance is needed to monitor the course of the HIV pandemic and plan appropriate public health responses. The World Health Organization (WHO) recommends HIV sentinel surveillance to monitor HIV infection. This is a method in which the prevalence of HIV is measured in specific populations whose blood has been obtained in the health care setting for other purposes and then tested for antibody to HIV only after the removal of all patient identifiers. For the surveillance of AIDS cases, WHO recommends the systematic reporting of AIDS cases using an appropriate national AIDS surveillance case definition. In light of developments in the understanding of the spectrum of severe HIV-related illness in both developed and developing countries, and the increased availability of laboratory diagnostic methods, however, a meeting was convened in Geneva by the WHO Global Program on AIDS to review the current case definitions for AIDS surveillance in adults and adolescents, and to make recommendations for their modification and application. The meeting recommended modifying the 1985 provisional WHO clinical case definition for AIDS, now to be referred to as the WHO AIDS surveillance case definition; and introducing an expanded WHO AIDS surveillance case definition. These two case definitions for AIDS surveillance are recommended for use in adults and adolescents in countries with generally limited clinical and laboratory diagnostic capabilities. They should therefore not be confused with clinical staging systems for HIV infection, which are useful for the clinical management of patients and for clinical research purposes. Case definitions for AIDS surveillance in children remain unchanged for now. The WHO case definition for AIDS surveillance, the expanded WHO case definition for AIDS surveillance, and comments are presented.