Surgical specimens (n = 48) or autopsy case materials (n = 15) were studied from 63 pediatric patients (44 males and 19 females) with intussusceptions involving the ileocecal junction (38 patients [60.3%]), ileum (16 patients [25.4%]), jejunum (four patients [6.3%]), and other sites (five patients [8%]). Lymphoid hyperplasia formed the leading edge in 32 cases (51%); other lesions included Meckel diverticulum (six cases), lymphoma (four cases), adenomyomatous hamartoma (four cases), cecal duplication cyst (three cases), ectopic pancreas (two cases), congenital bowel malformation (two cases), and examples of Peutz-Jeghers polyp, lymphangioma, leiomyoma, and inflammatory fibroid polyp (one case each). In six cases there was no associated lesion. Immunohistochemical evaluation for adenovirus was performed in 16 of the 32 cases in which lymphoid hyperplasia was present, and five reactive cases were identified; characteristic intranuclear adenovirus inclusions were visible on hematoxylin-eosin-stained specimens from all five of these cases as well as in five additional cases (a total of 10 of 32 cases [31.2%]). The presence of Yersinia sp was confirmed by serology in one case having characteristic histologic findings. Fourteen deaths were attributable to consequences of intussusception; these patients were younger (median and mean ages, 5.5 and 8.6 months; age range, 1 month to 3.5 years) than the surviving patients (median and mean ages, 2.0 and 3.2 years; age range, 6 days to 14 years), but were not more likely to have evidence of adenovirus infection.