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AIDS (acquired immunodeficiency syndrome) has been shown to be caused by the human immunodeficiency virus (HIV). However, the onset of the disease does not necessarily occur shortly after the person becomes infected. Infection by the virus itself is judged by the presence of specific antibodies that are produced by the body against the infection, and by the number of a particular class of lymphocytes (a type of white blood cell; the measurement is known as the CD4+ cell count). The rate of individuals developing AIDS after detection of HIV antibodies in their blood (seroconversion) has been studied in a group of individuals for whom the precise time of infection is known, namely hemophiliacs. Unlike individuals who have a high-risk lifestyle (e.g., homosexuals or drug users), patients with hemophilia usually become infected with HIV by medical treatment with contaminated human blood products; treatment records often allow determination of the time of infection. Out of a group of 1,219 individuals with hemophilia or similar conditions, 319 individuals for whom the date of HIV seroconversion was known were studied in detail. Statistical analysis of this group led to the finding that the risk of AIDS is directly related to age in individuals that seroconvert. The risk of an individual developing full-blown AIDS after infection is 2.67 per 100 person-years for the total population, 0.83 for persons between 1 and 11, and 5.66 for individuals 35 to 70. The common predictors, the number of CD4+ lymphocytes and a decrease in the quantity of antibodies produced against the virus (anti-p24 or anti- gp120), were valid indicators of increasing risk of AIDS in adults. Adolescents, on the other hand, had a lower rate of antibody loss, and a lower rate of AIDS after loss of anti-p24. There is thus evidence that the disease is more devastating among older individuals, and that children and adolescents are more resistant to developing AIDS. The article also speculates on the meaning of these findings, and suggests that the problems of AIDS are related to the re-initiation of previous infections that have been accumulated during a longer life.