Forty-two patients with idiopathic megacolon who were seen at St Mark's Hospital over an 18-year period are reviewed with regard to presentation, investigation and management. The differential diagnosis is discussed with particular reference to Hirschsprung's disease. The important aspects of medical treatment are outlined and the possible reasons for failure in more than one-half of the patients are discussed. The indications for surgical intervention in 14 patients are reviewed. The results following subtotal colectomy were better than following a localized sigmoid resection.