The incidence of Giardia lamblia infestation, as shown by examination of the stools, is the same in normal and coeliac children. 93 patients who were suspected of having coeliac disease had upper intestinal biopsies examined for giardia infestation. These patients usually had at least one stool and/or duodenal juice sample examined for G. lamblia. 26 patients (14 coeliac, 12 noncoeliac) had giardiasis. Only 13 patients would have been diagnosed if stools alone had been examined. Patients with giardiasis were usually male, younger than the other patients, more likely to have concurrent bacterial infection in the bowel, and more likely to come from a lower socioeconomic group. When the jejunal biopsies of 57 coeliac patients, including those with and without giardiasis, were compared, G. lamblia were more commonly found in those patients with less severe histological changes.