The death rate from tumours of the colon and rectum has risen rapidly in Japan since World War II. The rate of increase is greater in tumours of colon than in tumours of rectum. It is similar in males and females. It is large among the elderly, and this may be due to improvements in certification. But the difference between the Japanese and the US population is substantial at all ages. It is not known whether the current increase in the mortality of Japanese from tumours of the large bowel is due to differences between birth cohorts established early in life, or whether the differences developed after the end of World War II. Japanese who die of these tumours are younger than Americans, even when the age distribution of the populations is allowed for. This effect is produced by the differences between cohorts, and the age distributions within cohorts are closely similar in the two populations. The sex ratio for tumours of rectum rises with age in Japan in the same way that it does in the West, but the sex ratio of tumours of colon is apparently independent of age.