OBJECTIVE To compare the efficacy and acceptability of mebeverine and high-fibre dietary advice versus mebeverine and ispaghula in fixed combination in the treatment of irritable bowel syndrome in adults. DESIGN Open, prospectively randomised, parallel group comparison of mebeverine/dietary advice and mebeverine/ispaghula during an eight-week study period. SETTING General practices in the UK. PATIENTS One hundred and eleven patients with irritable bowel syndrome diagnosed by symptom profile or negative investigations between the ages of 18 and 75 years were entered. All patients had a history of abdominal pain occurring at least once a week for a period of three months or more. INTERVENTION Dosage was 135 mg of mebeverine hydrochloride, three times daily before meals, together with advice on high-fibre dietary intake, or 135 mg of mebeverine hydrochloride plus 3.5 g of ispaghula husk twice or three times daily before meals. MEASUREMENTS AND RESULTS Details of abdominal pain severity and frequency, bowel frequency and stool consistency were recorded by means of clinicians' assessments and patient diaries. Pre-treatment assessments revealed no significant differences between the two groups with respect to any of the parameters. Both treatment groups demonstrated highly significant improvements in the numbers of pain attacks and their severity; no statistically significant differences between the two groups were demonstrated. Five patients in the mebeverine/dietary advice group reported five concurrent effects and nine patients in the mebeverine/ispaghula group reported 13 concurrent effects. All of the mebeverine/dietary advice group found their treatment acceptable but up to 28% of the mebeverine/ispaghula group found their treatment unpalatable. CONCLUSION Both treatments are effective in the treatment of irritable bowel syndrome in adults. The fixed combination of mebeverine/ispaghula, however, was found to be unpalatable by up to 28% of the patients in that group. There does not, therefore, appear to be any advantage in using fixed combination therapy in this condition.