OBJECTIVE: Constipation is a common symptom affecting up to 30% of the Western population and is strongly associated with the presence of intestinal methanogens, which may directly inhibit motor activity. Two recent studies performed on adult and children affected by chronic constipation showed that the supplementation with L. reuteri significantly improved bowel movements. Whether its action is related to a decreasing of methane (CH4) production has never been tested. We have therefore designed a study aimed at testing this hypothesis. PATIENTS AND METHODS: Data of 20 adults (12 females, mean age 36.2 +/- 13.7) affected by functional constipation, treated with the probiotic L. reuteri (DSM 17938) for 4 weeks who performed a H-2/CH4 lactulose breath test (LBT) in our institution showing a CH4 production higher than 5 ppm were retrospectively analyzed from March to June 2015. Data recorded in their stool diary, reporting the frequency of defecations and stool consistency were also analysed, as well as the result of the LBT performed at the end of the treatment with L. reuteri. RESULTS: Four weeks of L. reuteri administration was associated with a significant decrease of mean CH4 production determined by LBT (from 20.8 +/- 15 to 8.9 +/- 8.6; p < 0.0001 CI 95%) and of AUC value (from 5101.5 +/- 3571.13 to 2128.4 +/- 2110.8; p < 0.0001 CI 95%). Moreover, a total disappearance of CH4 production (< 5 ppm at LBT) was observed in 11 patients, while, we did not observe any significant decrease of H-2 production (from 13.2 +/- 8.8 to 11.4 +/- 7.3, CI 95%, n.s.). CONCLUSIONS: This study highlights for the first time the beneficial effect of Lactobacillus reuteri (DSM 17938) on chronic constipation, via a significant decrease of CH4 production.