In humans, detailed studies of intestinal histopathology and function are provided only in case reports of immunocompromised patients, primarily with acquired immune deficiency syndrome. Diarrhea recurred in more than a third of the visitors with laboratory-confirmed infection and in 21% of residents with clinical disease. In some immunocompetent persons diarrhea can persist for a month, or rarely, as long as 4 months. In the National Health and Nutrition Examination Survey study, cryptosporidial IgG was detected in sera from 67.6% of persons 70 years of age and older. Although seropositivity might mean protection against clinical disease, this finding also suggests susceptibility to infection, although mostly asymptomatic, among individuals of advanced age. The epidemiologic and clinical features of cryptosporidial infection among the elderly, as well as the gastrointestinal and nongastrointestinal sequelae in immunocompetent hosts warrant further investigation. The relationship of the organism's genotype with clinical disease is yet to be defined.