Background and aims: To investigate the clinical effect of treatment with Lactobacillus acidophilus La-5 and Bifidobacterium animalis subsp. lactis BB-12 (Probio-Tec AB-25) to maintain remission in patients with ulcerative colitis. Methods: Patients with left-sided ulcerative colitis in remission - including proctitis and at least one relapse within the last year were randomised (2:1) in a double-blind placebo-controlled study to Probio-Tec AB-25 or placebo for 52. weeks. The patients were evaluated clinically, endoscopically and histologically at entry and if relapsing. No other medication for ulcerative colitis than the study drug was allowed during the study. Primary endpoint was maintenance of clinical remission, secondary endpoints comparisons of days to relapse, and safety and tolerability of the study drug. The concentrations of the probiotic bacterial strains in stool were analysed in a subset of patients. Results: Thirty-two patients were randomised. Twenty patients received Probio-Tec AB-25 and twelve patients received placebo. Five patients (25%) in the Probio-Tec AB-25 group and one patient (8%) in the placebo group maintained remission after 1. year of treatment (p = 0.37). The median time to relapse was 125.5. days (range 11-391. days) in the probiotic group and 104. days (range 28-369. days) in the placebo group respectively, (p = 0.683). Probio-Tec AB-25 was overall well tolerated. Conclusions: In this small randomised placebo-controlled trial no significant clinical benefit of Probio-Tec AB-25 could be demonstrated in comparison with placebo for maintaining remission in patients with left-sided ulcerative colitis. A difference may be achieved in larger studies, but the clinical significance of this would be questionable. This study was registered in ClinicalTrial.gov (NCT00268164).