Two patients who developed gastric giardiasis after 2 weeks of treatment with omeprazole 20 mg b.i.d. followed by omeprazole 20 mg per day for 2-6 weeks are described. In one patient, gastric giardiasis occurred in the presence of only mild intestinal metaplasia. In the other patient, Giardia lamblia infection had resolved 4 weeks after the cessation of omeprazole treatment, which, to our knowledge, is the first case reported in the literature. It is tempting to speculate that gastric giardiasis can develop in the absence of mucosal abnormalities during hypochlorhydria induced by treatment with a high dose of a proton-pump inhibitor. Specific treatment may not be required if the drug can be stopped and no other gastric disease causing hypochlorhydria is present.