AIM: To evaluate the prevalence of Giardia lamblia (G.lamblia) infection in patients with irritable bowel syndrome (IBS) and dyspepsia and to establish which is the most accurate test to diagnose the infection in this setting.METHODS: One hundred and thirty-seven patients who consecutively attended the Outpatient Gastroenterology Clinic for the first time between January 2002 and December 2003 due to symptoms of IBS and/or dyspepsia were recruited. All patients underwent clinical evaluation,first-step haematology and chemistry tests, serologic assays for celiac disease, lactose-H2 breath test, abdominal ultrasonography, and esophagogastroduodenoscopy.Helicobacter pylori status was evaluated. In patients with symptoms of IBS older than 45 years, colonoscopy was also performed. In all patients, duodenal biopsies and stool samples were examined for trophozoites and cysts of G.lamblia by several methods.RESULTS: G. lamblia was identified in 9 patients. The following diagnoses were also made: IBS (100/137, 73%), functional dyspepsia (62/137, 45%), organic dyspepsia (33/137, 24%), and lactose intolerance (75/137, 55%). A significant association was found between giardiasis and Hpylori infection (x2=6.632,OR= 12.4, CI= 1.5-68.1). There were no symptoms that reliably allowed the recognition of giardiasis.Direct search of the parasite in duodenal biopsy and stool sample examinations gave concordant results in all cases while histological examination of duodenal biopsies displayed a low sensitivity (e.g., 22.2%).CONCLUSION: In this consecutive series, diagnosis of G. lamblia infection accounted for 6.5% of patients with IBS and dyspepsia. Duodenal biopsies for diagnosis of giardiasis may be unnecessary if stool sample examination is performed.