In order to investigate etiologic distinctions among the anatomic sub-sites of the large bowel by sex, the relationship between large-bowel-cancer incidence and age at diagnosis, time period at diagnosis, and birth cohort was analyzed by anatomic sub-site and by sex, using data from the Connecticut Tumor Registry. Included in the study were all incident large-bowel-cancer cases occurring between 1950 and 1984 among Connecticut residents aged 40 to 79. Cancers of the large bowel were classified into 5 anatomic sub-sites: ascending colon (including cecum), transverse colon (including flexures), descending colon, sigmoid colon, and rectum (including rectosigmoid junction, anal canal, and anus). The data were fitted to log-linear age-period-cohort models. For each of the sub-sites, the age-period-cohort patterns for males and females differed. Within each sex, sub-site groupings with common patterns were indicated. Among males, the age-period-cohort patterns for the colon sub-sites were fairly similar; but the pattern for the rectum differed markedly from that for the colon sub-sites. There were secondary differences among the colon sub-sites that pointed to a secondary distinction between the right and the left colons. Among females, the age-period-cohort patterns for the left colon sub-sites and the rectum were fairly similar. The pattern for the transverse colon differed moderately from that of the left colon, and differed substantially from that of the rectum and the ascending colon. The ascending colon differed markedly from each of the other sub-sites. It is possible that these differences in age-period-cohort patterns reflect etiologic distinctions among sub-site groupings and between the sexes.