Giardiasis is one of the most common causes of infectious diarrhea and it has a worldwide distribution. Although it is a relatively common disorder, histologically confirmed Giardiasis is a rare occurrence and there have been no previous reports in Korea. A 34-year-old man presented with a 2-year history of epigastric discomfort and left flank pain. On stool examination, many cystic forms of Giardia lamblia were noted. In biopsy specimens of the small intestine, there was moderate chronic inflammatory cell infiltration and several trophozoites of Giardia were observed in the intervillous space and luminal surface. The trophozoites were pear shaped symmetrical organisms with two nuclei, and measured 9~10 micrometer in length.