More than 18 million persons in the world are estimated to have been infected with human immunodeficiency virus (HIV), the cause of the acquired immunodeficiency syndrome (AIDS). As immunodeficiency progresses, these persons become susceptible to a wide variety of opportunistic infections (OIs) The spectrum of OIs varies among regions of the world. Tuberculosis is the most common serious OI in sub-Saharan Africa and is also more common in Latin America and in Asia than in the United States. Bacterial and parasitic infections are prevalent in Africa; protozoal infections such as toxoplasmosis, cryptosporidiosis, and isosporiasis are also common in Latin America. Fungal infections, including cryptococcosis and Penicillium marneffei infection, appear to be prevalent in Southeast Asia. Despite limited health resources in these regions, some measures that are recommended to prevent OIs in the United States may be useful for prolonging and improving the quality of life of HIV-infected persons. These include trimethoprim-sulfamethoxazole to prevent Pneumocystis carinii pneumonia, toxoplasmosis, and bacterial infections; isoniazid to prevent tuberculosis; and 23-valent pneumococcal vaccine to prevent disease due to Streptococcus pneumoniae. Research is needed to determine the spectrum of OIs and the efficacy of various prevention measures in resource-poor nations, and health officials need to determine a minimum standard of care for HIV-infected persons. An increasing problem in the developing world, HIV/AIDS should receive attention comparable to other tropical diseases.