The effect of Plantago ovata on patients with chronic constipation (CC) with or without irritable bowel syndrome (IBS) has been assessed by a double blind study comprising 20 patients with CC of which 10 had associated IBS. A clinical questionnaire, weight of feces and intestinal transit time measured with radiopaque markers were done. Patients were then randomly distributed, 10 receiving PO and 10 placebo. Similar tests were done after treatment one month later. All patients receiving PO had good results against only one in the placebo group. Frequency of stools increased from 2.5 +/- 1 vs 8 +/- 2.2 stools per week, p less than 0.001 for paired data). A decrease in consistency of stools was also observed in the treated group. Fecal weight and colonic transit time were not significantly modified in placebo patients, while weight increase was observed in the treated ones (124 +/- 71 vs 194 +/- 65, gr/d p less than 0.001 for paired data) as well as a decrease in transit time (48 +/- 15 vs 34 +/- 18 hours p less than 0.05 for paired data). No adverse effects were observed and particularly no flatulence as often seen in patients on bran.