To determine the incidence and clinical features of Giardia lamblia infection, we studied 1790 patients at Kochi Medical School Hospital from April 1998 to July 2001. Fecal samples were examined microscopically by the direct smear method, direct immunofluorescent assay and by Kohn's one-step staining for G. lamblia cysts. Cysts of G. lamblia were found in 17 of 1,790(0.95%) stool samples, indicating that G. lamblia infection is not rare in Kochi. The most characteristic feature was that G. lamblia-positive cases were more frequent in the advanced age group(41-79 years old) and most of the subjects (except 2 cases) with G. lamblia had no history of traveling overseas. Four subjects had symptoms related to G. lamblia infection. Thus, more attention should be given to parasitic infections in laboratory stool examinations in order to detect cyst carriers as potential sources of infection.