Histological changes in 20 Giardia positive duodenal biopsies (Group A) were compared with 50, Giardia negative duodenal biopsies (Group B), taken during the same period. Stool examinations in Group B were negative for Giardia. Surface epithelium, villous and crypt architecture and cellular infiltrates were examined and compared between the groups. Atrophic changes in the villi were more common in Group A as compared to B(P < 0.0001). Intraepithelial neutrophil infiltration (P < 0.001), infiltration of the lamina propria with plasma cells (P < 0.5), and presence of eosinophils in the lamina propria (P < 0.001) were significant findings in group A. Some of the changes were related to the density of Giardia colonization e.g., the goblet cell depletion (P < 0.05) and the density of plasma cell infiltration in lamina propria (P < 0.01). Erosions and ulcerations were less commonly seen in group A. Thus we conclude that giardiasis manifests its peculiar features in the distal duodenal mucosa and a biopsy of this region is an important diagnostic tool for detection of this disease.