Epidemiological studies of ulcerative colitis (UC) have been conducted once every year since 1973 in Japan. The number of participating hospitals in 1429 and they are distributed throughout Japan. A total number of 10819 patients diagnosed with UC have been registered, including 197 patients with a positive family history. In this present study, genetic or environmental factors that may be operative in the development of UC were investigated. After 1966, the annual incidence of UC increased rapidly, to a peak in 1974, tending to remain at the same level since 1975. Since 1975, the daily intake of dairy products and meat increased, paralleling the increase in the incidence of UC. The trends of the increase in the incidence of UC and the increase in the daily intake of meat were very similar. The tendency for UC to occur in families was high, especially in first-degree relatives, and the age at onset of UC in patients where there was familial occurrence was younger. There was no difference in HLA antigens between UC patients with familial occurrence and the unaffected relatives studied. It is presumed that not only genetic factors but also environmental factors may be operative in the development of UC.