In the majority of individuals, infestation with trophozoites of Giardia lamblia (synonymous G. duodenalis or G. intestinalis) leads to a self-limited disease. Whereas most duodenal biopsies with chronic giardiasis show little or no inflammatory reaction, some patients may develop a severe disease with significant mucosal inflammation and various degrees of villous blunting. Occasionally, the histologic changes may resemble those of celiac disease. In this paper, we describe 11 patients, 5 of them female, with chronic giardiasis and demonstrable G. lamblia in ileal biopsies. The median age was 45 years (35 to 62 y), with male patients being at least 10 years younger than female patients. All of the duodenal biopsies showed at least mild villous blunting (grading: mild, marked, or total). In the mucosa an increased number of plasma cells and lymphocytes was observed. Furthermore, varying numbers of granulocytes were found in the lamina propria and in the epithelial layer. In 1 case only, the number of intraepithelial lymphocytes was >40/100 epithelial cells thus mirroring the histologic picture of celiac disease with a flat mucosa (with negative celiac disease-specific serological findings). Interestingly enough, all mucosal biopsy specimens from the duodenum were parasite free. Therefore, giardiasis could only be revealed by the demonstration of trophozoites of G. lamblia in biopsy specimens from the terminal ileum, which had been taken simultaneously or several weeks later. In contrast to duodenal biopsies, the ileal mucosa appeared either normal or only mildly inflamed in this setting. All patients but 1 were symptomatic, with chronic diarrhea being the leading symptom. Symptoms resolved after antibiotic therapy. This study demonstrates that giardiasis may be associated with a significant duodenal pathology in biopsy specimens without discernible parasites. In the cases described here infestation with G. lamblia was only proven histologically by examination of mucosal biopsy specimens taken from the terminal ileum.