Background Constipation is a prevalent gastrointestinal disorder. Patient dissatisfaction with prescribed medications is common, and there is need for alternative management strategies. Evidence shows that Bifidobacterium species may be beneficial in constipation. Aim To investigate changes in physiological and clinical measures of gut function in patients with chronic constipation following the consumption of Bifidobacterium lactis NCC2818, compared to placebo. Methods Participants were randomised to a 4-week supplementation with B. lactis NCC2818 (1.5 x 1010 CFU/d) or placebo. Gut transit time was measured using a radio-opaque marker, while symptoms and quality of life were assessed using validated questionnaires. Gut microbiota composition was assessed using quantitative polymerase chain reaction. Analysis of covariance was used for normally distributed variables, and Mann-Whitney test for non-normally distributed variables. Results Seventy-five participants were randomised. There was no significant difference between the probiotic and placebo groups in gut transit time change from baseline to week 2 (?11.7 hours, SD 33.0 hours vs ?12.9 hours, SD 33.6 hours; P = 0.863) or to week 4 (?20.4 hours, SD 32.5 h vs ?8.7 hours, SD 33.8 hours; P = 0.103). There were also no improvements in stool output, symptoms, or quality of life. No differences were found in Bifidobacterium concentrations between the probiotic and placebo groups at week 4 (9.5 log10/g dry faeces, SD 0.3 vs 9.4 log10/g, SD 1.0; P = 0.509). Conclusions Bifidobacterium lactis NCC2818 was not effective in the management of mild chronic constipation. This study highlights the importance of further studies and their publication to better understand the strain-specific effects of probiotics.