Changes in human immunodeficiency virus (HIV) infection among injection drug users (IDUs) are provoking a shift in the pattern of the HIV/AIDS epidemic in some regions of Brazil. IDUs living in 2 cities (Porto Alegre and Itajaı ́) in southern Brazil were contacted, interviewed, and tested for HIV. In 1998, 187 IDUs were surveyed and, in 2000, 352 IDUs. In Porto Alegre, HIV seroprevalence, age, and duration of injection history increased significantly over time. In contrast, a trend toward decline was observed in Itajaı ́. Homelessness, joblessness, and prison incarceration, as well as lifetime needle sharing, also increased over time in Porto Alegre. At the time of the study, at both sites, rates of needle sharing and condom use were similar, but use of syringe exchange programs, health and drug treatment, and HIV testing had increased over time. IDUs living in southern Brazil are at increased risk of HIV infection, indicating the continued need for permanent surveillance and preventive strategies.