Lymphocytic duodenitis and enteritis are conditions in which the number of intraepithelial lymphocytes is increased in areas away from the lymphoid follicles. IELs are phenotypically heterogeneous. Most are cytotoxic T cells. As already noted, IELs can be increased in a number of disorders, most notably celiac disease (Table 6.40). We use the term lymphocytic enteritis/duodenitis when there is an increase in IELs in the absence of what appears to be celiac disease or other known causes for the change. These patients often have chronic diarrhea and/or malabsorption. The lesion may be accompanied by lymphocytic gastritis and lymphocytic colitis. The IELs lack the atypia present in enteropathy-associated T-cell lymphoma. We do state the various entities in which intraepithelial lymphocytosis occurs in an effort to help the gastroenterologist narrow down a specific etiology. Some patients are eventually shown to have celiac disease, but in other cases the cause never becomes evident.