The first year of registration of colorectal tumors in a predominantly urban population (263,546 inhabitants) of northern Italy gave us the opportunity to investigate: (a) the incidence (crude, age-specific, age-standardized) of both colorectal cancer and polyps and their localization; (b) the familial occurrence of these neoplasms; and (c) if the data could fit into the "Adenoma-Carcinoma Sequence." Crude incidence of cancer was 52.8 new cases/100,000 in 1984, with 53.4 cases in men and 52.2 cases in women. The corresponding figures for polyps were 59.6 new cases, with 83.4 in men and 37.3 cases in women. The incidence increased with age for both cancer and polyps, although the latter were more frequent until patients were in their sixties and the peak of incidence of polyps anticipated that of cancer by a 5 year period. Both cancer and polyps had a similar distribution in the large bowel, more than 60% being located in the left distal portion. There were 72 cases of colorectal cancer among the first-degree relatives of the registered patients compared with 16 in the controls (RR = 4.26, chi 2 = 27.2 p less than 0.001). An increased frequency of cases of colorectal cancer in the families was found in both the cancer group and the polyp group. In conclusion, the observed incidence of large bowel tumors was similar to that of other well-developed countries. The earlier rise and peak of age-specific incidence of polyps as compared to cancer, the similar distribution of benign and malignant neoplasms in the various intestinal tracts, and the similar familial aggregation observed both in the cancer and in the polyp groups further support the "polyp-cancer sequence" and provide us with a promising strategy for the prevention of colorectal malignancies.