Intestinal neuronal dysplasia (IND) has been described proximal to the aganglionic segment in Hirschsprung's disease and less frequently as an isolated condition. Familial occurrence of IND is rare. We report a case of monozygotic twins affected with IND. One-year-old male monozygotic twins presented with a history of chronic constipation and abdominal distension since birth. At presentation, both twins had fecal impaction with palpable fecal-filled loops of bowel. Barium enema in both patients showed gross fecal loading of the sigmoid colon and rectum. Delayed films taken at 48 h showed marked barium retention. Suction rectal biopsy specimens examined by acetylcholinesterase (AChE) histochemistry in both patients showed moderately increased AChE-positive nerve fibers in the lamina propria, muscularis mucosae, and around submucosal blood vessels, as well as hyperplasia of submucous plexus with formation of giant ganglia. Both children underwent extended internal sphincter myectomy. Six months later, both patients had normal bowel function. The occurrence of IND in twins suggests that genetic factors are involved in the etiology of this disease.