fibre is effective in some types of constipation. Our objective was to determine if the administration of an enriched dairy preparation with a supplement of soluble fibre (Naturfibra) [inulin and digestion resistant maltodextrin (Fibersol 2)], improves primary chronic constipation.Prospective, randomized, double blind clinical trial randomized with dietary intervention in 32 subjects with constipation according to the Rome II criteria. Thirty two subjects (men 4; women 28) with an average age of 47 +/- 15 years were randomly divided in two homogeneous groups. A type of milk (A or B) was assigned to each group. Group A received fibre enriched semi-skimmed milk; Group B received semi skimmed milk. The subjects drank half a litre of milk per day during 20 days, meaning that those who drank the enriched milk ingested 20 grams of fibre a day.The subjects that presented straining at defecation (p < 0.001), sensation of incomplete evacuation (p < 0.001), sensation of obstruction in the evacuation (p < 0.001) and days between bowel movements, diminished significantly after the intake of milk supplemented with fibre.The intake of a dairy preparation with a supplement of fibre containing 20 grams of soluble fibre (inulin and digestion resistant maltodextrin), improves the situation of idiopathic primary chronic constipation based on the Rome II criteria.