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OBJECTIVES: Studies in the US and Europe have shown that appendectomy may prevent the development of ulcerative colitis, but no detailed study has been conducted in Japan, where ulcerative colitis is uncommon and the population is racially homogeneous. In addition, there has been no detailed analysis of the relationship between appendectomy and the clinical course of ulcerative colitis. In this multi-center clinical study, we evaluated the effects of appendectomy on ulcerative colitis in Japan. METHODS: A case-control study was undertaken in seven medical institutions comparing the incidence of appendectomy and tonsillectomy in 325 patients with ulcerative colitis and 325 controls matched for age (10-yr intervals) and sex. Disease duration, extent, and prognosis were determined in 21 patients with ulcerative colitis who underwent appendectomies and 304 patients with ulcerative colitis who did not undergo appendectomies. RESULTS: Appendectomy was performed in a significantly lower percentage of patients in the ulcerative colitis group (21/325, 6.5%) than in the control group (53/325, 16.3%) (p < 0.001) (odds ratios = 0.355, 95% CI = 0.208-0.603). In contrast, no significant difference was observed between the groups with respect to tonsillectomy. The mean age (25.7 ± 10.9 yr) of patients with ulcerative colitis at the time of appendectomy was significantly higher than the mean age (20.1 ± 8.7 yr) of patients in the control group at the time of appendectomy (p < 0.05). The incidence of proctitis was higher in the appendectomy group than in the group that did not undergo appendectomies (38.1% vs 18.1%). In addition, the recurrence rates were significantly lower in the appendectomy group than in the group that did not undergo appendectomies (57.1% vs 78.6%, p < 0.05), although both groups were similar in composition as to sex, age, duration disease, smoking status, and previous medical treatment. CONCLUSIONS: Our results indicate that appendectomy has negative association with and perhaps a preventive effect the development of ulcerative colitis in the Japanese population. Furthermore, appendectomy also appears to reduce the extent and recurrence of ulcerative colitis. [ABSTRACT FROM AUTHOR]