The safety and efficacy of the sustained release (SR) mebeverine capsule was compared to the standard plain mebeverine capsule in the treatment of 60 patients suffering from irritable bowel syndrome. Patients, with a score of at least 44 on the Kruis scale, were randomized into a two-period crossover trial. Each treatment period lasted for 6 weeks during which the patients took mebeverine plain 135 mg, two capsules t.i.d., or mebeverine sustained release 200 mg (SR), two capsules b.i.d. After 6 weeks of each treatment, both treatments were regarded 'effective' or 'very effective' by the patient as well as the investigator in more than 80% of the cases. After 3 weeks of the first treatment, the disease score was rated light in 73% of the patients with both medications. After 6 weeks, nine patients (33%) were symptom-free with mebeverine plain, and five (18%) with mebeverine SR. During the second treatment period the number of symptom-free cases reached about 40% with both formulations. Considering the clinical general improvement, more than 70% of all patients had improved after 3 weeks of the first treatment. An additional improvement was reported in 13 patients with mebeverine plain and in 10 patients with mebeverine SR after the next 3 weeks. Abdominal pain was still present in more than 50% of patients but with lower intensity compared with baseline values. Mean scores of efficacy were very similar for both treatments after 3 and 6 weeks (2.0 for mebeverine plain vs. 1.9 for mebeverine SR). The statistical comparison of all scores between the two formulations did not show a significant difference at any time. Very few adverse events were noted and a causal relationship with the study medications was judged as improbable or definitely unrelated. Compliance was close to 100% for most of the patients. The results of the present study indicate that the mebeverine SR capsule provides equivalent efficacy and tolerance to mebeverine plain in the treatment of irritable bowel syndrome (IBS), while reducing the number of daily doses from three to two.