The anatomic site distribution of large bowel cancer was studied in 2079 patients between 1967 and 1980. To measure the trend in the localization of the carcinomas, the slope of the regression line of the proportions at each site over the years was computed. The large bowel was divided into five segments: cecum, ascending colon and hepatic flexure, transverse and descending colon, sigmoid and rectosigmoid junction, and rectum. A significant decrease in the proportions of cancer in the sigmoid for both sexes (slope parallel b = -0.89; P less than 0.001) and an increase of cecal cancer (b = 0.54; P less than 0.004) was observed. Sex-specific results indicated the diminishing proportion of sigmoid cancer in men (b = -1.04; P less than 0.004) with an increase in transverse and descending colon cancer proportions (b = 0.52; P = 0.012). In women, a negative slope of -0.74 was not significant for sigmoid cancer, but cancer of the cecum showed an increase in proportions (b = 0.80; P = 0.01). The age distribution indicated a significant decrease in proportion of men older than 80 years and of women in the age group of 40 to 49 years and an increase in women older than 80 years. However, age adjustment did not change any of the previous conclusions. The observed sex differences in the changing distribution within the large bowel over a 14-year period cannot be explained by the improvement of diagnostic tools in the last years. In view of these findings, it is important to evaluate the possible sex-related bias in the application of screening and preventive measures as well as the changes in the ecologic features of the large intestine.