OBJECTIVES: The aim of this study was to investigate the characteristics of colonic mucosal lesions by colonoscopy and for assessment of clinical features in patients with acute diarrhea. METHODS: From March 2000 to August 2001, one hundred and ten patients with watery or bloody diarrhea lasting less than 14 days. The colonoscopic finding was assessed to extent of lesion(rectum, left, right and total colon) and severity of inflammation(mild ; erythema and edema, moderate ; exudates and blood coagula, severe ; hemorrhage and ulcer). RESULTS: Male was 60(54.5%) and median age was 43 years(range ; 15-84 years). The day of colonoscopy from symptom onset was 5 days(range 1-14 days). The causative food was flesh and meat, sea food and fish, and vegetable in order. Eight patient have history of travel including oversea. Pancolitis was the most common finding(46/110, 41.8%) and the normal colonoscopic finding was noted in eighteen cases(16.4%). The mild inflammation was 40(43.5%), moderate was 26(28.3%) and severe was 26(28.3%). The severity of inflammation was significantly correlated to the extent of lesion(p<0.05, r=0,655). CONCLUSIONS: The colonoscopy in acute infectious colitis was a useful to evaluate the extent of lesion and the severity of disease.