Background: A sensation of abdominal swelling (bloating) and actual increase in girth (distension) are troublesome features of irritable bowel syndrome (IBS), which is more common in patients with constipation, especially those with delayed transit. Aim: To establish whether a fermented dairy product containing Bifidobacterium lactis DN-173 010 reduces distension in association with acceleration of gastrointestinal transit and improvement of symptoms in IBS with constipation. Methods: A single centre, randomized, double-blind, controlled, parallel group study in which patients consumed the test product or control product for 4 weeks. Distension, orocaecal and colonic transit and IBS symptoms were assessed on an intention-to-treat population of 34 patients. Results: Compared with control product, the test product resulted in a significant reduction in the percentage change in maximal distension [median difference - 39%, 95% CI (-78, -5); P = 0.02] and a trend towards reduced mean distension during the day [-1.52 cm (-3.33, 0.39); P = 0.096]. An acceleration of orocaecal [-1.2 h (-2.3,0); P = 0.049] as well as colonic [-12.2 h (-22.8, -1.6); P = 0.026] transit was observed and overall symptom severity [-0.5 (-1.0, -0.05); P = 0.032] also improved. Conclusions: This probiotic resulted in improvements in objectively measured abdominal girth and gastrointestinal transit, as well as reduced symptomatology. These data support the concept that accelerating transit is a useful strategy for treating distension. © 2008 The Authors.