Abstract Background & Objectives: Giardia lamblia is one of the most common protozoa in human being throughout the world. The different isolates of this parasite have various potential pathogenicity. Clinical demonstrations of Giardiasis are acute diarrhea, chronic diarrhea syndrome and malabsorbtion.The aim of this study has been the observation of the different kinds of clinical features in patients with Giardia lamblia referred to the Hamadan faculty of medicine, 2004- 2005 Materials & Methods: The fecal samples of 274 patients referred to Hamadan faculty of medicine were examined via direct and Formol- Eter methods. By a questionnaire, the subjects’ clinical manifestations were recorded and then analyzed. Results: In this study, 20.4% of the patients (55.4% male, 44.6% female) were infected with Giardia. 46.5% of them complained of abdominal pain. The prevalence of Giardia was 30.8 %( 6-10 years) and 23.1% (1-5 years). The most common signs observed were abdominal pain, diarrhea and vomiting, respectively. Conclusion: The most common sign observed in this study was abdominal pain, partially similar to studies carried out in other parts of Iran and the world too. Key word: Giardiasis, Clinical Signs,Epidemioloy,Hamadan