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In most instances, constipation is considered idiopathic or functional. The total and segmental colonic transit time, traced by radio-opaque markers, makes possible the identification of the colon segment that has the motility alteration that causes constipation.A study was performed of 13 adolescents, aged 12 to 18, with functional chronic constipation and 13 without constipation. In all of them the total and segmental colonic transit times were measured with radio-opaque markers. The adolescents ingested 20 markers each on three successive days, and on the fourth day a plain abdominal radiograph was performed.In the nonconstipated adolescents the total colonic transit time (mean± SD) was 30.2 ± 13.1 hours, in the right colon 5.7 ± 3.9 hours, in the left colon 7.9 ± 7.8 hours, and in the rectosigmoid 15.5 ± 10.6 hours. In the constipated adolescents, the total colonic transit time was 58.3 ± 17.4 hours, in the right colon 15.9 ± 12.4 hours, in the left colon 14.7 ± 13.4 hours, and in the rectosigmoid 17.2 ± 16.2 hours. There was a statistically significant difference (p < 0.05) in the total colonic transit time, and in both the right and left colon transit times between constipated and nonconstipated adolescents.The measurement of total and segmental colonic transit times is a simple method that allows one to distinguish constipation due to colonic dysfunction(right colon and left colon) from constipation due to distal obstruction(rectosigmoid).