The outcome in 40 patients who underwent colectomy for idiopathic megacolon and megarectum over an 18 year period was evaluated. All patients had a radiologically dilated bowel and a bowel frequency of less than two per week. Twenty two patients had a caecorectal anastomosis, 11 had an ileorectal anastomosis (including one with a previous caecorectal anastomosis and four with a previous sigmoid resection), and seven had a sigmoid resection. The mean (range) age at operation was 35 (17-69) years. All three operations resulted in a normal bowel frequency in more than 80% of patients but no patient with an ileorectal anastomosis experienced recurrent constipation. Thirty four patients experienced pain preoperatively and this was still present in 14 patients postoperatively. One patient died and four required subsequent laparotomy for bowel obstruction. The functional outcome in patients with dilatation of the whole colon and in those with dilatation of the left colon did not differ. Subsequent surgery for constipation was performed in three patients. Colectomy offers good results with few complications in the treatment of idiopathic megacolon, and an ileorectal anastomosis is the preferred operation.