The present pilot trial was carried out to evaluate the effects of an acute treatment with a mixture containing 500 million of Lactobacillus bulgaricus and Streptococcus thermophilus per day in patients with non alcoholic fatty liver disease (NAFLD). A sample of 30 patients with NAFLD (diagnosed by liver biopsy) was enrolled and 28 patients were analyzed in a double blind randomized clinical trial. Patients were randomized to one of the following treatments during 3 months: group I, treated with one tablet per day with 500 million of Lactobacillus bulgaricus and Streptococcus thermophilus and group II, treated with one placebo tablet (120 mg of starch). In group I, alanine amino transferase (ALT: 67.7 +/- 25.1 vs. 60.4 +/- 30.4 UI/L; p < 0.05), aspartate aminotransferase activity (AST: 41.3 +/- 15.5 vs. 35.6 +/- 10.4 UI/L; p < 0.05) and gammaglutamine transferase levels (gammaGT: 118.2 +/- 63.1 vs. 107.7 +/- 60.8 UI/L; p < 0.05) decreased. In group II, all liver function parameters remained unchanged (ALT: 60.7 +/- 32.1 vs. 64.8 +/- 35.5 UI/L; p < 0.05), aspartate aminotransferase activity (AST: 31.7 +/- 13.1 vs. 36.4 +/- 13.8 UI/L; ns) and gammaglutamine transferase levels (gammaGT: 82.1 +/- 55.1 vs. 83.6 +/- 65.3 UI/L; ns). Anthropometric parameters and cardiovascular risk factors remained unchanged after treatment in both groups. A tablet of 500 million of Lactobacillus bulgaricus and Streptococcus thermophilus, with a randomized clinical design, improved liver aminotransferases levels in patients with NAFLD.