Gastrointestinal illness are among the most common and debilitating complication of infections with HIV, affecting 50 per cent to almost 100 per cent of AIDS patients in developed and developing countries, respectively. A number of factors including relevant modes of transmission, the environment, and immunosuppression conspire to determine which enteric infectious agents HIV-infected persons acquire. In developed countries, transmission of a diverse spectrum of bacteria, viruses, and protozoa is facilitated by unprotected receptive and intercourse and anal-lingual contact among homosexual men with multiple partners. In developing countries,l where most HIV infections occur among heterosexual persons, waterborne and foodborne transmission are the principal modes of transmission of enteric organisms. The severity and duration of symptoms associated with enteric pathogens are determined by the host's immunologic response to the organism. Candida albicans often causes local mucosal disease but less often causes systemic infections in HIV-infected persons, likely because polymorphonuclear cell function is intact. The ability of AIDS patients to control infections with G. lamblia and C. jenuni is related to their ability to mount an antibody response to these organisms during infection. The virulence of the organism may also affect the clinical response to infection. Cryptosporidium causes diarrheal symptoms in both immunocompetent and AIDS patients, but illness is more severe and prolonged in the latter. Giardia lamblia and C. jejuni infections are associated with a range of clinical manifestations in both AIDS patients and HIV-seronegative persons, whereas CMV and possibly adenovirus appear to cause significant disease only among immnocompromised subjects. The availability of effective therapy is among the most decisive factors in determining the duration of enteric infections in AIDS patients. For example, Giardia lamblia may cause acute abdominal pain and diarrhea in HIV-infected subjects but prolonged infections with the parasite are uncommon because effective therapy is available. In contrast, infections with CMV and Cryptosporidium may be severe anc chronic as available therapy is generally ineffective or only transiently effective. Awareness of these clinical, epidemiologic, immunologic, and therapeutic aspects of gastrointestinal illness in HIV-infected subjects should help to direct the diagnostic evaluation of these patients and to direct areas of research.