A woman with Peutz-Jeghers syndrome is reported in whom malignant transformation was found in a polyp of the small intestine. The patient was operated on for occlusion of the small intestine. A tumour of the jejunum was disclosed and histological examination revealed, besides typical structure of a Peutz-Jeghers polyp, also severe carcinomatous dysplasia with beginning invasion of the submucosa. The modern strategy of management of Peutz-Jeghers polyps is discussed.