Background and purpose. Modic changes (MC) are a risk factor for development of chronic low back pain (CLBP). There is no agreement about the cause of inflammation in MC, but autoimmunity has been suggested. The aim of the study was to investigate whether treatment with lactic acid bacteria for 100 days was associated with change of disability and pain, via a change in the gut microbiota inducing a change in the immune system, in patients with CLBP and type 1 MC during one year follow-up. Methods. Eighty-nine patients with CLBP and type 1 MC were randomized to receive either one capsule Lactobacillus Rhamnosis GG or placebo capsules twice daily for 100 days. Results. Missing values at one year were 4% and 3% in the disability and pain variables, respectively. The predefined outcomes disability and back and leg pain only changed little during follow-up with no statistically significant differences between groups. At one year, back pain had decreased by 1.1 more on a 0–10 scale (95% CI 0.20- 1.97) in the experimental group than in the control group. There were no differences regarding other predefined outcomes, i.e. global effect or percentage with minimal disability at one year. Nine percent of the patients reported gastrointestinal side-effects without difference between groups. Conclusions. No differences were found between groups regarding the predefined outcomes. Overall, the study confirmed that CLBP with MC1 is a grave back pain disorder, with little tendency to improvement. During follow-up, disability of the whole cohort was reduced by just 17%. Conflicts of interest: No conflicts of interest. Sources of funding: The study has been supported by The Danish Rheumatism Association and Peter and Helga Kornings Fond