A case of Peutz-Jeghers syndrome in which one colonic polyp showed many foci of glandular dysplasia is reported. Peutz-Jeghers polyps are considered to be hamartomatous, and their malignant potential has been considered doubtful. This case is valuable because foci of glandular dysplasia existed within hyperplastic glands. Glands of Peutz-Jeghers polyps are proliferative in nature, and therefore it is not surprising that there are areas of glandular dysplasia and of cancerous change.