Patients with the Peutz-Jeghers syndrome carry a slight, though definite, increased risk of gastrointestinal carcinoma. The malignant potentiality of Peutz-Jeghers hamartomatous polyps, generally considered benign, is supported by this report. Two cases of metastasising gastrointestinal carcinoma associated with the Peutz-Jeghers syndrome are described in a 56 year old female and her 29 year old son. Both mother and son died from duodenal and gastric carcinomas respectively, which developed in hamartomatous polyps with extensive metastases. Both cases also showed dysplastic areas within hamartomatous polyps. These features indicate that hamartomatous polyps may, in some cases, be the precursors of digestive tract carcinomas.