PURPOSE:This study was designed to investigate neural and muscular features of the colonic wall in patients with severe idiopathic constipation.METHODS:By using quantitative immunohistochemistry, resected specimens from 14 patients with idiopathic chronic constipation and 17 nonobstructed cancer controls were studied.RESULTS:Routine histology revealed no significant histologic abnormality throughout the colon apart from four cases of melanosis coli. Ratio of the thickness of circular to longitudinal muscle was significantly lower in the left colon in constipated subjects. The myenteric plexus appeared morphologically normal in all subjects. S‐100 protein, which stains neuronal supporting tissues, demonstrated an increase in the proportion of neural tissue in the myenteric plexus. There was an increased number of PGP‐9.5 immunoreactive nerve fibers in the muscularis propria in constipated patients, and this was significantly higher in the ascending and descending colon.CONCLUSION:Intractably constipated patients have alterations in the neural composition of the colonic myenteric plexus and innervation of the circular muscle.