Recurrent aphthous stomatitis (RAS) is a frequent condition characterized by recurrent and painful oral ulcers with unknown pathophysiology. Recent studies suggest that a dysregulation of the oral microbiota may be implicated. Currently, therapies for RAS are limited by severe side effects or inconstant effectiveness. The aim of this study is to assess the effectiveness and safety of a probiotic, the Lactobacillus rhamnosus Lcr35® , in the treatment of RAS. A placebo-controlled, parallel study will be conducted in 40 subjects with RAS. Treatment consisted on the administration of the daily probiotics or placebo during 3 months.All patients will be then followed up for additional 3months without treatment. The main outcome measure will be the number of occurring aphtae.