Clostridioides difficile infection (CDI) is the most common cause of nosocomial diarrhea, and the most common health care-associated infectious disease in the United States, accounting for 15% of overall infections, nearly 30.000 deaths per year an estimated economic expense of $5 billion/year. In the last decade, most of the burden related to CDI depends on recurrence CDI (rCDI) (3). rCDI is known to extend the hospitalization length, and to be associated with increased morbidity and mortality rates. Furthermore, rCDI is often, more than primary infection, associated with life-threatening complications, including pseudomembranous colitis, toxic megacolon, shock, perforation, bloodstream infection (BSI), sepsis, caused by intestinal bacteria or fungi with a mortality rate nearly 50%, and death. Fecal microbiota transplantation (FMT), defined as the infusion of feces from healthy donors to recipient with disorders associated to dysbiosis, is known to be a highly effective treatment option against CDI. FMT is also more effective than standard treatment with vancomycin and it is recommended by International Guidelines for treating multiple recurrence of CDI. Despite the increasing body of evidence about the clinical efficacy of FMT for the treatment of rCDI, mechanisms for this clinical efficacy are also unknown. Metagenomics analysis is known as a good option to examine gut microbiota and to estimate microbial diversity. The aim of this study is to evaluate changes in microbial composition in rCDI patients after FMT, using metagenomics analysis.