Increasing incidence of cecal and sigmoid carcinoma. Data from the Connecticut Tumor Registry. We have studied both the distribution and incidence of colorectal cancer using The Connecticut Tumor Registry, the oldest tumor registry in the United States. During the time period 1973 to 1985, left-sided colon cancers accounted for 63% of the cancers, right-sided cancers 33%, and cancers with unspecified sites 4%. Indeed, this pattern of distribution has remained constant for 25 years. For the period 1935 to 1985, we calculated the sex-specific, age-adjusted (normalized to the 1970 U.S. Census) incidence. Age-adjusted incidence of rectal cancer has remained stable for 50 years: for men, 22.8 cases/100,000/year, and for women, 13.9 cases/100,000/year. During these 50 years, the age-adjusted incidence of cecal carcinoma for men has increased from 3.6 to 16.7 cases/100,000/year, while for women, it has increased from 4.9 to 14.2 cases/100,000/year. Sigmoid carcinoma for men has increased from 8.8 to 18.7 cases/100,000/year, and for women, it has increased from 7.7 to 12.8 cases/100,000/year. The incidence of colon cancer at each site has been and continues to be increasing at a constant rate. Age-adjusted incidence for all colorectal cancers has increased from 35.2 to 70.2 cases/100,000/year for men and from 32.1 to 49.2 cases/100,000/year for women. Thus, distribution of colorectal cancers by site in Connecticut has remained stable for 25 years. More importantly, however, the age-adjusted incidence of colon cancer has continued to increase for 50 years, whereas that of rectal cancer has remained relatively stable. 