Adult patients with an idiopathic megarectum or megacolon can experience severe constipation requiring surgical treatment. Some of these patients have a proximal colon of normal diameter, with dilatation involving only the left or distal colon and rectum. The results of partial colonic and rectal resection with coloanal anastomosis in such patients have been reviewed. Seven patients (two female and five male) underwent a coloanal anastomosis over a seven-year period. The median age at operation was 19 years, the mean age at onset of symptoms was five years, and the mean follow-up period was one year. Five patients experienced a return to normal bowel frequency with the loss of most symptoms. One patient has an ileostomy because of persistent constipation after the procedure. One subject died because of postoperative bleeding from the anastomosis and subsequent cardiac and respiratory complications. This operation may have a place in the treatment of severe constipation caused by idiopathic megarectum and megacolon, but careful preoperative motility studies and meticulous attention to operative technique are required for a good outcome.