BACKGROUND: Generally ulcerative colitis has a character that has a continuous pathological lesion from the rectum toward the cecum. Ulcerative appendicitis with a skipped appendiceal orifice lesion, which is unusual in ulcerative colitis, has been infrequently reported, and its clinical characteristics have not been identified. PURPOSE: This study was carried out to evaluate the incidence rate and the clinical characteristics of ulcerative appendicitis. METHODS: One hundred consecutive patients with ulcerative colitis who had been treated from Jan. 1997 to Aug. 1998 at Song-Do Colorectal Hospital were used for the study. Data evaluated included age, sex, involved site, clinical type, clinical severity of the disease, and endoscopic severity of the disease. RESULTS: Nineteen (19%) of the 100 patients had skipped lesions around the appendiceal orifice; the other 81 did not. There were no significant differences between these two groups with respect to the age and the sex distributions, the involved site, the clinical type, and the clinical severity. There was a correlation between the endoscopic grades, based on the Riley classification, of the lesions at the rectum and at the appendiceal orifice. Seven patients (36.8%) of the 19 patients with appendiceal orifice lesions showed an extended lesion from the appendiceal orifice to the cecum. CONCLUSIONS: We suggest that appendiceal lesions in ulcerative colitis are not infrequent. Even though no significant differences in the clinical characteristics of ulcerative colitis with ulcerative appendicitis, compared with those of ulcerative colitis without ulcerative appendicitis, were found, we suggest that more profound study of ulcerative appendicitis probably contribute to understand the pathophysiology of ulcerative colitis.