After discussing the importance of developing accurate statistical systems for monitoring HIV infection, the chapter discusses aspects of the prevalence and incidence of HIV infection, including principles and methods of estimating prevalence. The chapter then describes the current (1989) HIV surveillance system developed and managed by the Centers for Disease Control (CDC), which consists of six components called the 'comprehensive family of surveys.' Overall, the assessment concludes that the surveys cannot characterize (with knowable margins of error) the prevalence or incidence of HIV infection in any well-defined population. The chapter recommends that the CDC reformulate the family of seroprevalence surveys as probability samples and that it institute a continuing anonymous probability survey of the HIV serostatus of women who are clients of clinics that provide abortion services. It also recommends that the newborn infant seroprevalence survey be extended to include all children born in the United States. The assessment concludes that what is required for more informative debates, better planning for future health care needs, and improved evaluation of the effects of national AIDS-control strategies are data derived from research designs that can provide reasonably unbiased estimates of the prevalence and incidence rates for HIV infection in well-defined populations of substantive interest. 38 footnotes, 28 references.