Using pathology reports and other data from the Cancer Surveillance Program, the population-based cancer registry of Los Angeles County, we evaluated demographic characteristics and the detailed subsite distribution of adenocarcinomas of the small intestine for the period 1972-1985. The most striking finding was the great preponderance of these tumours in the duodenum, especially in comparison with other histologic types of small bowel cancers. Fifty percent of all small intestinal adenocarcinomas occurred at this location, even though the duodenum comprises just 4% of the entire length of the small bowel. Furthermore, after excluding those cases occurring in the duodenum but with indeterminate subsite, 57% of these duodenal primaries could be mapped to the 2nd portion of the duodenum, a six to seven centimeter segment containing the Ampulla of Vater. We could pinpoint the location for 48 of the 77 tumours (62%) occurring in this segment, specifically to areas adjacent to the Ampulla. We also confirmed the high male to female ratio of small bowel adenocarcinomas in blacks and non-Latino whites, but could find no evidence of such an effect in Latinos or Asians; however, the number of cases was not large in these latter two racial-ethnic groups.