INTRODUCTION: Melatonin (MEL) exerts beneficial effects on the gut partly by myorelaxative properties upon the smooth muscle. Its secretion decreases with age, particularly in postmenopausal women. This study was aimed at evaluating the effect of MEL on the symptoms of irritable bowel syndrome (IBS) in this group of patients. MATERIAL AND METHODS: The investigations were carried out in 80 postmenopausal women, aged 48-65 years, divided into two equal groups, diagnosed according to Rome Criteria III: i.e. patients with IBS with constipation predominant (IBS-C), and patients with IBS with diarrhoea predominant (IBS-D). The control group (C) included healthy women aged 46-65 years. In all subjects, 6-sulfatoxymelatonin (6-HMS) concentration urine was measured using ELISA assay. Patients in both groups over the course of six months were given melatonin (at a dose of 3 mg fasting and 5 mg at bedtime) or a placebo (double blind trial). Disease activity was evaluated after two, four and six months, using a ten-point scale to assess the main somatic symptoms: visceral pain, abdominal bloating, etc. RESULTS: The amounts of 6-HMS urine excretion (μg/24 h) were: C 11.4 ± 3.0, IBS-C 10.2 ± 3.2, IBS-D 14.0 ± 6.3 (p 〈 0.05). Correlation between values of symptoms score and contrary excretion of 6-HMS: IBS-C r = -0.714, IBS-D r = 0.409. After six months in the IBS-C group, the intensity of visceral pain and abdominal bloating had decreased in 70% of patients (p 〈 0.01) and constipation in 50% of patients (p 〈 0.05). Beneficial changes in the IBS-D group were noted in 45% of patients, but this was not better compared to the placebo. CONCLUSIONS: Melatonin can be used as part of the treatment of IBS, particularly in patients with constipation-predominant IBS.