The authors studied 51 patients who had small bowel tumors that contained adenomatous epithelium. These rare lesions were identified among 392,000 surgical pathology cases seen during a 62-year period. Grossly and histologically, adenomas arising in the mucosa of the small intestine are similar to the adenomas found in the colon. Of the 51 patients, 18 had adenomas, and 33 had tumors that contained both adenoma and carcinoma in the same lesion, including five intramucosal and 28 invasive carcinomas. The location of the tumor usually determined which clinical problems were produced. The data indicate that adenomas originating in the small bowel mucosa probably are premalignant lesions and that many primary adenocarcinomas of the small intestine arise in adenomas. Of the authors' 130 apparently primary small bowel carcinomas (including the papilla of Vater), 33 (25%) histologically demonstrated adenomatous epithelium in the same lesion. Factors associated with an increased chance of finding carcinoma in an adenoma include adenoma type, size of lesion, location, and multicentricity. Carcinomas appear to develop more frequently in papillary (villous) adenomas than in ordinary adenomas. The larger the lesion is, the more likely carcinoma will be identified. Adenomas involving the ampulla contain carcinoma more often than do lesions found elsewhere in the duodenum and small intestine. Three patients had multiple adenomatous polyps of the small bowel; two of these individuals also had duodenal carcinomas. Various problems in pathologic diagnosis and clinical management are discussed.