The case of an 84-year-old woman with evidence of continuous gastrointestinal bleeding is presented. A barium study and endoscopy showed a 25 x 20 mm sized semipedunculated polyp in the second portion of the duodenum. No other lesion was found throughout the entire gastrointestinal tract, and endoscopic resection of the duodenal polyp was performed. The resected polyp showed typical histologic features of a Peutz-Jeghers type hamartomatous polyp, although the patient lacked any familial history and signs related to the Peutz-Jeghers syndrome. A focus of adenocarcinoma was also found within the polyp. The present case would appear to be the first demonstration of a solitary Peutz-Jeghers type hamartomatous polyp of the duodenum containing a focus of adenocarcinoma.