One hundred and five appendices removed at the time of surgical reduction of intussusception in children were studied by light microscopy after routine procedures to search for aetiological factors involved in intussusception. Normal pediatric appendix specimens served as controls (n = 30). Light microscopic examination showed viral inclusions in epithelial cells in 48 of 105 appendices (45%) from the cases of intussusception. No viral inclusion was observed in controls. Immunohistochemistry performed on 21 appendices from intussusception with a monoclonal antibody against adenovirus showed intranuclear positivity in all appendices with viral inclusions. Viral inclusions seen in epithelial cells of appendices from cases of intussusception are caused by virus and in particularly by adenovirus. The etiological factors involved in intussusception without viral inclusion in appendix remain unknown.