The authors report the unique case of a patient with the Peutz-Jeghers (P-J) syndrome who had multiple small bowel hamartomatous polyps, some of which also showed adenomatous and carcinomatous changes. Over the course of 30 years, over 100 small bowel and colonic polyps were resected; all histologically demonstrated the typical features seen in hamartomatous polyps. Several jejunoileal polyps excised during the last few months of the patient's life showed not only hamartomatous features but also adenomatous epithelium. As far as the authors can determine, this is the first documented case of adenomatous changes found within a small bowel hamartomatous polyp. In addition, at least one duodenal polyp demonstrated areas of hamartoma, adenoma, and in situ carcinoma. The literature on the occurrence of gastrointestinal adenomas and carcinomas in patients with the P-J syndrome is reviewed, and the relationship of hamartomas, adenomas, and carcinomas of the gastrointestinal tract is discussed.