Parasitic causes of diarrhea are common in pediatric patients and have important public health implications. Therefore, diagnosis should be pursued vigorously in the appropriate clinical setting. Giardia and Cryptosporidium infections are the most common causes of disease in the United States, and stool examination for ova and parasites is the best way to make the diagnosis. This should be performed three times, on alternate days, in order to rule out parasitic disease with confidence. In treatment of Giardia, it is important to interrupt transmission, particularly in the day-care setting. Because of the potential for major outbreaks, patients with cryptosporidiosis should be kept out of day care until their symptoms resolve. Although amebiasis is not a common problem in the United States, all patients with bloody diarrhea who have traveled to, or are from, endemic areas should be tested for amebiasis by serology and stool examination. If patients with suspected inflammatory bowel disease are being considered for therapy with corticosteroids, mucosal scrapings of colonic lesions should be examined for amoebae, because corticosteroid therapy can lead to more invasive amoebic disease. Acquired immunodeficiency syndrome patients with diarrhea can be infected with a number of pathogens and a specific diagnosis should be pursued aggressively. These patients are often infected with one or more of the parasites described in this article. The approach to diarrhea in these children is described in a separate article of this issue (Deveikis A. 'Gastrointestinal Disease in Immunocompromised Children," pages 562-569).