The efficacy, tolerance and acceptability of lactulose (Duphalac) and ispaghula (Fybogel Orange) were assessed in the treatment of chronic constipation in adults. In an open, prospectively randomised, parallel group study, 124 patients with a history of constipation for more than three weeks were treated with either 15 ml bd of lactulose (increasing to 60 ml daily if necessary) or one sachet bd of ispaghula. Over the four-week treatment period both treatments were shown to be effective, and numbers of concurrent effects were similar between the two groups. Differences were demonstrated with regard to acceptability in favour of lactulose.