It is still unclear how to evaluate the existence of faecal retention or impaction in children with defaecation disorders. To objectivate the presence and degree of constipation we measured segmental and total colonic transit times (CTT) using radio-opaque markers in 211 constipated children. On clinical grounds, patients (median age 8 years (5-14 years)) could be divided into three groups; constipation, isolated encopresis/soiling and recurrent abdominal pain. Barr-scores, a method for assessment of stool retention using plain abdominal radiographs, were obtained in the first 101 patients, for comparison with CTT measurements as to the clinical outcome. Of the children with constipation, 48% showed significantly prolonged total and segmental CTT. Surprisingly, 91% and 91%, respectively, of the encopresis/soiling and recurrent abdominal pain children had a total CTT within normal limits, suggesting that no motility disorder was present. Prolonged CTT through all segments, known as colonic inertia, was found in the constipation group only. Based on significant differences in clinical presentation, CTT and colonic transit patterns, encopresis/soiling children formed a separate entity among children with defaecation disorders, compared to children with constipation. Recurrent abdominal pain in children was in the great majority, not related to constipation. Barr-scores were poorly reproducible, with low inter- and intra-observer reliability. This is the first study which shows that clinical differences in constipated children are associated with different colonic transit patterns. The usefulness of CTT measurements lies in the objectivation of complaints and the discrimination of certain transit patterns. Conclusion. Abdominal radiographs, even when assessed with the Barr-score proved unreliable in diagnosing constipation.(ABSTRACT TRUNCATED AT 250 WORDS)