The clinical findings of a population-based colorectal tumor registry have been analyzed to determine elements of supporting or not supporting the existence of different types of large bowel cancer. Age-specific incidence rate of the 409 registered patients rose sharply with increasing age in all segments of the large bowel; however, regarding left colon and rectum, the male: female ratio showed a marked male preponderance, more evident in the more advanced age groups. Histopathology, studied in 87% of patients, revealed adenocarcinoma as the most frequent feature; however, adenocarcinoma with concomitant adenoma (i.e., presumably arising in adenoma) was observed in 14.3% of cancers of the left colon, in 17.7% of rectal tumors, but in only 5.7% of neoplasms of the proximal colon (P less than 0.05 and P less than 0.01, respectively, vs. left colon and rectum). Some histological features (carcinoid and mucinous carcinoma) were observed in right-side tumors only. Analysis of the familial occurrence of cancer showed that a significantly larger proportion of patients with neoplasms located in proximal colonic segments had three or more first-degree relatives affected by (or deceased from) cancer of all sites. Similarly, colorectal tumors among relatives were more frequent in patients with right-side cancer. The location of the 793 polyps observed during 3 years of registration showed that more than 70% of adenomas were located beyond the splenic flexure, overlapping the distribution of cancers. In conclusion, the differences of sex ratio at different colonic subsites, the higher fraction of adenocarcinomas with adenomas in cancer of the more distal tracts of the large bowel, and the more marked familial occurrence of colorectal cancer in patients with right-side neoplasms tend to support the view that cancer of the proximal colon, cancer of the distal colon, and cancer of the rectum may actually be three different types of tumors.