In the United States, approximately 1 million Americans are infected with human immunodeficiency virus (HIV), and several thousand new infections are reported each year. More than one-third of cases of acquired immunodeficiency syndrome (AIDS) are associated with injection drug use. An estimated 1.8 million adults and children are currently living with HIV in Latin America and the Caribbean, and injection drug abuse remains a major factor in initial exposures to HIV in these parts of the world. Although only 3 cases of AIDS related to drug abuse have been reported in Bolivia, a country with a nascent epidemic, >19,000 cases of AIDS have been reported in Argentina and >22,000 in Brazil, with a significant number associated with injection drug use. Extensive epidemiological and clinical research has been conducted in the United States and elsewhere to determine the extent and nature of the problem and to design and develop interventions (prevention and treatment) for drug abusers infected with HIV. The articles in this supplement present a current view of the nature and extent of the bloodborne and sexually transmitted infections in drug abusers and their partners in the Western Hemisphere.