Megacolon is an abnormal ilatation of the colon (a part of the large intestines) that is not caused by mechanical obstruction. The dilatation is often accompanied by a paralysis of the peristaltic movements of the bowel, resulting in chronic constipation. In more extreme cases, the feces consolidate into hard masses inside the colon, called fecalomas (literally, fecal tumor), which require surgery to be removed.