We aimed to investigate the value of scintigraphically determined evacuation fraction (EVF) as an objective follow-up criterion in medically treated children with chronic constipation, and to evaluate the relation between patients' symptoms and rectal emptying rate. Thirty children (m: 22, f: 8, mean age: 8.9+/-0.9 year) suffering from chronic constipation were assessed by scintigraphic evacuation study. All patients underwent anal manometric and scintigraphic examinations before their therapy was started. During 3 months, they received conventional laxative treatment for constipation, and scintigraphic studies were repeated in 20 of 30 patients. Scintigraphic examinations consisted of instillation of artificial stool into the rectum up to the volume impending rectal evacuation, and acquisition of pre- and post-evacuation images. EVF values were then calculated from the images using a fixed region of interest. All of the patients had high resting anal canal pressure (75+/-12 mmHg) and had positive rectoanal inhibitory reflex. In 14/20 patients (70%) who benefited from therapy, mean EVF changed from 43.6% to 62.2% (p<0.001), while decreasing in 6/20 who did not benefit. The volume arousing rectal evacuation was significantly lower at the:second scintigraphic study (261+/-98 mi) than the first study (354+/-138 mi) (p<0.05). As a conclusion: 1) EVF was low in chronically constipated children, 2) it was improved after conventional laxative treatment and, 3) radionuclide EVF determination may be a useful method for follow-up.