The intestinal mucosa is an important portal of entry of HIV, and HIV-infected mononuclear cells are found in the intestinal lamina propria of 30 to 50% of HIV-infected patients even at early stages of the disease. HIV infection of epithelial cells has not consistently been detected and is still controversial. Intestinal T cells are phenotypically and functionally distinct from circulating T cells, especially in their state of activation and differentiation, which both affect the replication and cytopathicity of HIV. An increase in CD8+ cells and variable decreases in CD4+ cells have been found in the intestinal lamina propria by immunohistologic studies, resulting in a decreased CD4 to CD8 ratio; in addition, CD25 expression is reduced. Changes in intraepithelial lymphocytes are unclear. B-cell differentiation seems to be disturbed because IgA plasma cells and IgA2 secretion are reduced. Depletion or functional impairment of activated mucosal lamina propria lymphocytes by HIV infection could explain the breakdown of the mucosal immune barrier leading to opportunistic diseases; in addition, due to the interrelationship between mucosal immune system and epithelium, these changes might be responsible for the partial bowel atrophy and maturational defects in enterocytes of HIV-infected patients.