Ultrastructural studies of the small and large intestine from children revealed five cases showing abnormal small electron-dense bodies within enterocytes and goblet cells. The characteristic features of the bodies were: (a) they occurred within microvilli and within the apical part of the cell cytoplasm; (b) they showed an association with the most apically located desmosomes of the epithelial cells; (c) in the microvilli they were present within protrusions of the membrane, suggesting that they may bud out of the cell; and (d) they were not membrane-bound within the cell but were found outside the cell as luminal, membrane-bound particles. These bodies occurred in two children with Crohn's disease and in three children having other gastrointestinal disorders. Similar electron-dense bodies which did not have all of the characteristics listed above were found in a sixth child who had Crohn's disease. These electron-dense bodies do not resemble any known inclusion of intestinal epithelium, nor do they resemble any previously described microorganism, although their apparent release from the cell by a process of budding is suggestive of a microbial identity. The occurrence of these bodies in childhood Crohn's disease suggests that they may be disease-related, but their precise nature and relevance to this or other gastrointestinal disorders remain unknown.