Histological changes in duodenal biopsy specimens from 80 patients infected with Giardia lamblia were defined and compared with changes in duodenal biopsy specimens from 80 randomly chosen "healthy" patients. Villous architecture, the number of intraepithelial lymphocytes, the occurrence of lymphoid follicles and the density of colonisation by G lamblia trophozoites were examined. Grade 1 villous flattening was observed in 41% of cases positive for Giardia and in 37.5% of the control cases. The mean content of intraepithelial lymphocytes was 24.5 compared with 22.6 intraepithelial lymphocytes/100 epithelial cells in the positive and negative groups, respectively. Lymphoid follicles were more frequently observed in Giardia positive specimens, but their incidence was not significantly higher compared with that of controls. Twenty five per cent of patients positive for Giardia showed light, 41% intermediate, and 34% heavy colonisation by Giardia trophozoites. In 76% the parasites were observed in all specimens obtained. It is concluded that histological changes induced by G lamblia are not specific, and that two duodenal biopsy specimens might suffice to detect the trophozoites.