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infection and to plan appropriate public health responses. The revised clinical staging and immunological classification of HIV are designed to assist in clinical management of HIV, especially where there is limited laboratory capacity. The final revisions outlined here are derived from a series of regional consultations with Member States in all WHO regions held throughout 2004 and 2005, comments from public consultation and the deliberations of a global consensus meeting held in April 2006. In most countries, reporting of acquired immunodeficiency syndrome (AIDS) cases has been incomplete and children are rarely included. Further, timely and appropriate use of antiretroviral therapy delays and may prevent the development of AIDS as previously defined. The advances in antiretroviral therapy (ART) therefore mean that public health surveillance of AIDS alone does not provide reliable population-based information on the scale and magnitude of the HIV epidemic. Data on adults and children diagnosed with HIV infection are more useful for determining populations needing prevention and treatment services. Simplified HIV case definitions are provided based on laboratory criteria combined with clinical or immunological criteria. The clinical staging of HIV-related disease for adults and children and the simplified immunological classification are harmonized to a universal four-stage system that includes simplified standardized descriptors of clinical staging events. The revised HIV case definitions and the clinical and immunological classification system proposed are intended for conducting public health surveillance and for use in clinical care services. WHO recommends that national programmes review and standardize their HIV and AIDS case reporting and case definitions in the light of these revisions. In 1986, WHO developed a provisional clinical AIDS case definition for adults and children (Bangui definition) to report AIDS cases in resource-constrained settings . The definition was formalized in 1986 and modified in 1989 (for adults and adolescents only) to include serological HIV testing and then modified again in 1994 to accommodate 1993 revisions to European and United States Centers for Disease Control and Prevention definitions. European and United States Centers for Disease Control and Prevention definitions include specific case definitions for children. Studies in African settings suggest that the original WHO clinical case definitions for AIDS in children are not very sensitive or specific. AIDS case reporting in middle- and low-income countries has been incomplete and of variable accuracy, which has hampered its utility. Underreporting and delays in notification are frequent and exacerbated by weak heath information systems and the lack of diagnostic capacity. In high income countries, AIDS case reporting combined with active AIDS case-finding has allowed AIDS notification and AIDS specific mortality to be monitored. However, the widespread availability of successful antiretroviral therapy means that both new AIDS cases and AIDS mortality have been declining in countries with high coverage of antiretroviral therapy The scale-up of services for ART, preventing mother-to-child transmission of HIV (PMTCT) and HIV counselling and testing has led to an increase in the numbers of adults and children being tested and diagnosed with HIV infection. Accurate data are needed on adults and children diagnosed with HIV infection to facilitate estimation of the treatment and care burden, to plan for effective prevention and care interventions and assess care interventions. WHO therefore recommends that countries consider conducting reporting of newly diagnosed cases of HIV infection in adults and children (Box 1). The requirements for the confidentiality and security of HIV surveillance data are the same as for AIDS-related reporting. Provider-initiated reporting will be required to increase the completeness, timeliness and efficiency of HIV case reporting. Laboratory-initiated reporting alone will be insufficient for reporting HIV, as other surveillance information from the health care provider or medical records will be required. For the purposes of HIV case definitions for reporting and surveillance, children are