- Summary Giardia lamblia is the most common protozoan isolated from the gastrointestinal tract. Giardiasis affects predominantly children and young adults, and its usual symptoms are abdominal pain, diarrhea, and malabsorption. Interestingly, the intestinal mucosa may appear unremarkable even in clinically severe giardiasis. In addition, the protozoan Giardia lamblia is notorious for its inconspicuous appearance in hematoxylin and eosin sections. Moreover, it can still be missed even with the use of special stains such as Giemsa or trichrome. In the present study, we investigated the immunoreactivity of Giardia lamblia to c-kit in duodenal mucosal biopsies. Giardia lamblia trophozoites were immunoreactive for c-kit in all of the study group cases (n = 14) and in none of the control group cases (n = 10). Thus, c-kit immunostain is a very sensitive method for the demonstration of Giardia lamblia. Moreover, the protozoan attains a conspicuous and unique appearance, whether it floats freely in the lumen between the duodenal villi, adheres to the mucosal surface, or is located in the cytoplasm of the epithelial cells. We recommend performing c-kit immunostain in every duodenal biopsy with clinical or pathologic suspicion of giardiasis.