Intranuclear inclusion bodies were found by light microscopy in epithelial cells in more than one-third of the specimens from children operated on for ileocecal intussusception. Electron microscopic examination done on hematoxylin and eosin-stained slides showed the intranuclear inclusion bodies to be composed of viral particles in large and small crystalline arrays. Adenovirus of serotypes 2, 3, and 5 were isolated from the five cases with inclusions in which isolation was attempted. These findings strongly suggest a pathogenetic role for adenovirus in those cases of intussusception in which intranuclear inclusion bodies are found in the epithelial cells of the appendix or the terminal ileum.