Congenital colorectal innervation defects were evaluated by studying 3699 colonic mucosal biopsy specimens obtained from 773 patients over a 5-year period (1986–1991). In 358 cases (46.3%) a classifiable defect was present, with aganglionosis in 187 of these patients (52.2%) and hypoganglionosis of the colon in 18 (5.0%). Hypoplastic or aplastic sympathetic innervation (type-A neuronal intestinal dysplasia was found in 2.2% (n=8) and dysplasia of the parasympathetic submucous plexus (type-B neuronal intestinal dysplasia) in 40.6% (n=145) of the patients with classifiable defects. Identification of a specific innervation defect was not possible in 229 of the 773 patients (29.6%), 28% of whom exhibited slight dysplasia and 30% immaturity or hypogenesis of the submucous plexus. In 40% of the unclassifiable cases heterotopic nerve cells were found in the muscularis mucosae and/or lamina propria mucosae, while 2% had severe heterotopia with the cells of the myenteric plexus completely displaced into the circular and/or longitudinal muscle layers. These patients generally suffered from severe chronic constipation requiring surgical intervention. Four congenital innervation defects of the colorectum can thus be clearly differentiated at present: aganglionosis (in its various forms), hypoganglionosis, type-A neuronal intestinal dysplasia, and type-B neuronal intestinal dysplasia.