Background: Children with developmental disorders experience chronic gastrointestinal symptoms. Aims: To examine the nature of these gastrointestinal symptoms and histologic findings in children with autism spectrum/developmental disorders and ileocolonic disease. Methods: Chart review. 143 autism spectrum/developmental disorder patients, with chronic gastrointestinal symptoms, undergoing diagnostic ileocolonoscopy. Results: Diarrhea was present in 78%, abdominal pain in 59% and constipation in 36%. Ileal and/or colonic lymphonodular hyperplasia (LNH), defined as the presence of an increased number of enlarged lymphoid follicles, often with hyperactive germinal centers, was present in 73.2%. Terminal ileum LNH presented visually in 67% and histologically in 73%. Colonic LNH was multifocal and presented histologically in 32%. Ileal and/or colonic inflammation presented in 74%, consisting primarily of active or chronic colitis (69%). Ileal inflammation presented in 35%. Presence of LNH significantly predicted mucosal inflammation. Patients with ileal and/or colonic LNH had lower mean/median age than those without; patients with ileal and/or colonic inflammation had lower mean/median age than those without. There was a significant association between ileo and/or colonic inflammation or LNH, and onset of developmental disorder; plateaued or regressive onset conferred greater risk than early onset. Conclusions: Patients with autism or related disorders exhibiting chronic gastrointestinal symptoms demonstrate ileal or colonic inflam - mation upon light microscopic examination of biopsy tissue. Further work is needed to determine whether resolution of histopathology with appropriate therapy is accompanied by GI symptomatic and cognitive/behavioral improvement.