During 1980-1985, 304 children with functional megacolon were examined. Group I was made up of 95 patients with chronic constipation; group II of 130 patients with associated megacolon and encopresis; group III of 54 patients who mainly complained of abdominal pain (the mean age was 5.2, 7.5 and 8.8 years, respectively); group IV included 25 children (19.6%) with megacolon recognized from the group of 127 patients in whom acute appendicitis was excluded according to irrigoscopy. In the children of the first 3 groups, the x-ray signs of colitis or irritated colon were identified in 54, 72 and 87%, respectively. These groups may in fact illustrate the natural time-course of changes in the disease development. It has been demonstrated that with age functional megacolon may be aggravated by colitis. A pathogenetic diagram of the development of functional megacolon is provided.