An ultrastructural study was performed to identify any differences in fine detail between type III and IV gastric polyps, which seem to differ from each other because they have different histological features and malignant potential. Clear ultrastructural differences were found, implicating microvilli, mucous granules, interdigitations, ribosomes, mitochondria and nuclei. Although these two subtypes have often been grouped together as borderline lesions or adenomas, the findings imply that they should be treated as essentially different.