HIV infection increases non-Hodgkin's lymphoma and Kaposi's sarcoma risk. Among HIV-uninfected persons, risk for these malignancies and others increases with age. As HIV-infected persons age, new patterns in cancer incidence may emerge. In this article, data from the AIDS-Cancer Registry Match study are presented on risk for Kaposi's sarcoma and lung cancer among persons with AIDS. For 132,346 homosexual men with AIDS, Kaposi's sarcoma incidence was highest for men 30-39 years old (5.0 cases/100 person-years) and declined with age (P(trend) <.0001). This trend likely arises from variation in Kaposi's sarcoma herpesvirus prevalence among homosexual men. For 239,257 adults with AIDS (all risk groups), lung cancer incidence increased with age, and was higher than in the general population (P <.0001), probably reflecting heavy smoking among HIV-infected adults. Identifying separate effects of HIV and aging on cancer risk will require detailed data on individuals' HIV infection status and exposures to known carcinogens.