Purpose: The prevalence of inflammatory bowel diseases is much lower in Asian countries, including Japan, than in Western countries, but it is rapidly increasing. However, no recent reports describe the current prevalence of these diseases in Japan, so we performed a descriptive epidemiological study to remedy this situation and to elucidate various characteristics of inflammatory bowel diseases in this country. Methods: Japan has a nationwide registration system of patients with intractable diseases, including ulcerative colitis and Crohn's disease. To calculate the age-standardized prevalence, we used this registration system to collect patient data, and we obtained detailed population data from the Japanese government's population estimates made in 2003 and 2004 and from the 2005 population census. In addition, information about the characteristics of ulcerative colitis and Crohn's disease patients was collected through the registration system. Results: The age-standardized prevalence of ulcerative colitis in Japan in 2005 was 63.6 per 100,000 persons, and that of Crohn's disease was 21.2. Patient numbers have been steadily increasing with time. The age distribution was found to differ between the two diseases, with Crohn's disease affecting mainly younger people. In both diseases, more than 50% of the patients were male, and over 80% of the patients were classified as mild to moderate in terms of severity. Conclusions: The prevalence of inflammatory bowel diseases in Japan is still much lower than in Western countries. Surveillance should be continued, and research to clarify their etiologies in association with the increasing number of patients in Japan is needed.
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