Data from nine population-based cancer registries participating in the Surveillance, Epidemiology, and End Results Program from 1973 through 1982 were analyzed to determine the demographic correlates of the incidence of the four major histologic types of cancer of the small intestine: carcinomas, malignant carcinoid tumors, lymphomas, and sarcomas. These tumors were uncommon: The average annual incidence per million persons was 3.9, 2.9, 1.6, and 1.2, respectively. The age-specific incidence rate for each type was quite low through middle age, after which it rose sharply. Males experienced higher rates than females for each of the four histologic types. Blacks were at a 40-70% greater risk than whites for the occurrence of carcinomas and carcinoid tumors but at a 50% lower risk for lymphomas. The reported incidence of malignant carcinoid tumors increased during the decade under study, the rate at the end of the interval being 50% greater than that at the beginning. The extent to which this increase represents improved detection of these tumors or a change in the criteria separating malignant from benign tumors could not be evaluated. There was no change in the incidence of carcinomas, lymphomas, or sarcomas of the small intestine. The four histologic types varied considerably with regard to their proportional distribution among the subsites of the small intestine. Carcinoid tumors, lymphomas, and sarcomas rarely occurred in the duodenum, whereas nearly half of the carcinomas were found there. Eighty-seven percent of the carcinoid tumors and 60% of the lymphomas occurred in the ileum. Clues to the etiologies of tumors of the small intestine are sparse. The incidence data presented here suggest that at least some of the etiologies will differ among the histologic types.