Background Probiotics are live microorganisms administered primarily in fermented milk products that increase the viability of the microbiota, allow the physiological processes of the intestine and modulate the immune system.The probiotic L. casei Shirota has decreased the inflammatory response in mice with collagen-induced arthritis. Objectives To determine the effect of Lactobacillus casei Shirota on the clinical manifestations, gastrointestinal symptoms and serum levels of pro and anti-inflammatory cytokines: TNF-α, IL-1 b and IL-10 in patients with active RA. Methods We performed a controlled single blinded clinical trial that included 19 female patients with active RA (DAS >3.2), ≥18 years of age, with no other inflammatory rheumatic o infectious disease and on stable treatment (DMARDs and prednisone <10 mg/d) in the last 3 months. They were randomized to two groups: the study group was given fermented milk at a dose of 4×1010 CFU of L. casei Shirota per day for 6 weeks and a placebo group. Cutoffs were made at 0, 6 and 9 weeks, the latter as a washout period to see if the effect remained. DAS28-ESR, HAQ-DI, gastrointestinal symptoms with the validated Gastrointestinal Symptom Rating Scale questionnaire (GSRS) and serum levels of cytokines: TNFα, IL-1β and IL-10 (ELISA) were evaluated. Statistical analysis was performed with SPSS using nonparametric tests Kruskal Wallis and Mann-Whitney test. Results We included 11 patients in the study group (51±12 years of age) and 8 patients in the placebo group (54±20 years of age). No statistical significance in clinical findings was observed between both groups, but patients in the study group did improvein HAQ DI values that decreased from 1.03±0.4 at week 0 to 0.58±0.4 and 0.51±0.5 at week 6 and 9 respectively (p<0.05). A favorable trend was observed in DAS28, 5.5±0.8, 5.2±0.9 and 4.9±1.1 at week 0, 6 and 9 respectively. An improvement in gastrointestinal symptoms was seen with a GSRS score of 8.4±4.8 at baseline that decreased to 3.8±2.3 at week 9 (p<0.05). Proinflammatory cytokine levels also tended to decrease and an increase in anti-inflammatory cytokines was seen but without statistical significance between baseline and end of study results, but neither when compared with the placebo group. No patient experienced side effects. Conclusions These results suggest that probiotic L. casei Shirota may have beneficial additive effect when used to treat RA. However more studies are needed with a higher dose, longer period of time and a larger amount of patients to determine effects of L. casei Shirota. References So JS, Kwon HK, Lee CG, Yi HJ, Park JA, Lim SY, et al. Lactobacillus casei suppresses experimental arthritis by down-regulating T helper 1 effector functions. Mol Immunol. 2008 May;45(9):2690-9. Disclosure of Interest None Declared