Giardia lamblia is a ubiquitous intestinal protozoan. Transmission, which is fecal-oral, occurs after cyst ingestion, excystation and enterocyte adhesion. Symptoms include diarrhea, abdominal pain and other less frequent manifestations such as nausea, anorexia and weight loss. Enzyme immunoassay and direct fluorescent-antibody assay for antigen detection in stool samples, and small intestine biopsy provide the best diagnostic sensitivity. When an infection is suspected, duodenal biopsy should be carried out, even though endoscopic appearance may be normal. The most effective drugs in the treatment of this infection are metronidazole and tinidazole. We report a case of G. lamblia infection. The patient reported a clinical history of 4 months' duration with general malaise and diarrhea for the previous month. This form of presentation is rarely found in adults with this parasitosis. The diagnosis was based on the results of duodenal biopsy. The patient showed satisfactory response to treatment with metronidazole.