Chronic ulcerative colitis is a systemic inflammatory disease with uncertain etiology primarily involving the colonic mucosa. The mucosal biopsy interpretation is important for an evaluation of the disease state and further medical or surgical treatment. However, few clinical and pathological studies of the endoscopic diagnosis of this disease are available in Korea. Therefore, we evaluated the clinical and pathological characteristics of it diagnosed by endoscopic biopsy and analysed the reasons for the discrepancy between clinical and pathologic diagnosis for a more accurate endoscopic mucosal biopsy diagnosis in the future. A total of 702 cases of colonic mucosal biopsy specimens during Feb. 1994 and Jan. 1995 at Severance hospital, Yonsei University College of Medicine were reevaluated for the study. A clinical diagnosis of ulcerative colitis, after endoscopic examination, was made in 61(8.7%) cases. A pathological diagnosis was made when there is an increased inflammatory cell infiltration in the mucosa with evidences of a chronic crypt injury in the biopsy specimens. Using this criteria, a diagnosis was made in 32(52.3%) cases. In 29 cases the diagnosis was made in the first biopsy specimen and in the remaining 3 cases the diagnosis was made in the second or third biopsy specimens. No pathologic diagnosis of ulcerative colitis was made in the cases that clinical diagnosis was not. In the 32 cases diagnosed as ulcerative colitis, 14 cases were involved the rectum and sigmoid colon, 9 cases were involved up to the descending colon, 1 case was involved up to the transverse colon and 8 cases showed pancolonic involvement. In 29 cases, which ulcerative colitis was suspected clinically but was not consistent with it pathologically, 8 cases were proved to be ischemic colitis, 5 cases were acute infectious colitis and one case was Crohn's disease by repeat examination and follow up. Ten cases were histologically within normal range and lesions subsided spontaneously with no recurrence. A conclusive diagnosis could not be made in 5 cases during this study period. From these results, we conclude that ulcerative colitis can be diagnosed accurately by endoscopic biopsy, and clinical follow up and repeat examination are valuable in the differential diagnosis of this disease.