BACKGROUND/OBJECTIVES: Obesity and metabolic disorders are linked to inflammation via gut microbiota and/or gut permeability. Gut-derived endotoxin triggers inflammation leading to metabolic syndrome (MetS) and contributing to oxidative stress. We intended to investigate the effect of Lactobacillus casei Shirota on gut permeability, presence of endotoxin and neutrophil function in MetS. SUBJECTS/METHODS: Patients with MetS were randomized to receive 3 × 6.5 × 10⁹ CFU L. casei Shirota (probiotic group) or not for 3 months. Gut permeability was assessed by a differential sugar absorption method and by determination of diaminooxidase serum levels, endotoxin by an adapted limulus amoebocyte lysate assay, neutrophil function and toll-like receptor (TLR) expression by flow cytometry and ELISA was used to detect lipopolysaccharide-binding protein (LBP) and soluble CD14 (sCD14) levels. RESULTS: Twenty-eight patients and 10 healthy controls were included. Gut permeability was significantly increased in MetS compared with controls but did not differ between patient groups. None of the patients were positive for endotoxin. LBP and sCD14 levels were not significantly different from healthy controls. High-sensitive C-reactive protein and LBP levels slightly but significantly increased after 3 months within the probiotics group. Neutrophil function and TLR expression did not differ from healthy controls or within the patient groups. CONCLUSIONS: Gut permeability of MetS patients was increased significantly compared with healthy controls. L. casei Shirota administration in the MetS patients did not have any influence on any parameter tested possibly due to too-short study duration or underdosing of L. casei Shirota.