Constipation in childhood is a common symptom, with an estimated incidence between 0.3% and 8%. Most of the evidence for the current management of constipation and fecal soiling in children is based on reports of nonrandomized retrospective trials. Anal dilatation has had an established role in the management of idiopathic constipation but has never been evaluated by a randomized study. A double-blind randomized controlled trial was done of children who failed to respond to medical treatment and were admitted for investigation and treatment of idiopathic constipation to Guy's Hospital, London, between April 2001 and April 2003. All children had intestinal transit study on admission. They were randomized, using a computer-generated allocation in sealed envelopes, to receive no anal dilatation (control group) or anal dilatation (anal dilatation group). Anorectal manometry and endosonography were done under ketamine anesthesia followed by anal dilatation if necessary under the same anesthesia. Disimpaction of feces from the rectum was done at the end of the procedure under general anesthesia using propofol muscle relaxant to minimize stretching of anal sphincter muscles in the control group. All children had intensification of medical treatment, toilet training, and monitoring of their response to treatment during their hospital stay, which ranged from 3 to 5 days. Outcome was measured using a parent's questionnaire of symptom severity at 3 and 12 months of follow-up by one of the authors, who was blinded to randomization. The symptom severity score ranged between 0 and 65 and consisted of scores for the following: delay in defecation (score range 0-10), difficulty and pain with passing stool (0-5), soiling problem (0-10), intensity of laxative treatment (0-10), child's general health (0-5), behavior related to the bowel problem (0-5), overall improvement of symptoms (0-12,) and assessment of megarectum on abdominal examination (0-8). Of 60 neurologically normal children, 31 (19 males) were randomized in the control group and 29 (18 males) in the anal dilatation group. All children had findings consistent with idiopathic constipation and positive anorectal reflex on manometry, no anal sphincter damage on endosonography, and no anal fissure on examination under anesthesia. The median age for control and anal dilatation groups was 7.97 (range 4.1-14.25) years and 7.78 (4-13.25) years, respectively. Both groups were also comparable with regard to median of duration of laxative treatment (32 months vs. 31.5 months), internal anal sphincter thickness on endosonography (0.90 mm vs. 0.80 mm), resting anal sphincter pressure on manometry (51 mmHg vs. 51 mmHg), total rectal capacity on manometry (260 mmHg vs. 260 mmHg), and total symptom severity score before admission (33 vs. 29), respectively. At 12-month follow-up, the median pre-admission symptom severity score had improved significantly, from 33 (range 12-49) in the control group and 29 (16-51) in the dilatation group to 15 (0-51, p < 0.0001) and 19 (1-46, p < 0.0001), respectively. There was no significant difference between the two groups with regard to symptom severity score improvement at 12-month follow-up (p < 0.92). We found a significant correlation between total rectal capacity measured on manometry and symptom severity score before admission and at 12-month follow-up (r = 0.30, p < 0.01 and r = 0.25, p < 0.05, respectively). Our results indicate that anal dilatation does not contribute to the management of school-aged children with idiopathic constipation. Admission to hospital for clarification of diagnosis and intensification of medical treatment with disimpaction of stool from the rectum is beneficial.