To clarify the potential for malignancy of Peutz-Jeghers polyposis, we investigated 75 gastrointestinal polyps resected surgically or endoscopically from seven patients with this syndrome.There were 19 polyps in the stomach, 18 in the duodenum, 22 in the small intestine, and 16 in the large intestine, and these were histologically composed of 1 pyogenic granuloma, 1 cancer in adenoma, 2 adenomas, and 71 Peutz-Jeghers polyps. Nine of these Peutz-Jeghers polyps were accompanied by an adenomatous component, and, in addition, two of these showed a cancerous transformation with stalk invasion. A total of 12 neoplastic polyps (16 percent) were found in three relatively young patients (aged 20, 25, and 43 years), all of which were pedunculated and located either in the duodenum or in the jejunum. There was no statistical significance in size between the neoplastic polyps (mean +/- SD, 20.1 +/- 10.8 mm) and the completely hamartomatous polyps (mean +/- SD, 15.8 +/- 9.0 mm). Moreover, the configuration of these types of polyps seemed similar.Neoplastic transformation is not a rare event, and our results may indicate evidence of a hamartoma-adenoma-carcinoma sequence in Peutz-Jeghers polyposis.