whereby FMT exerts its sometimes miraculous benefi ts in patients with recurrent Clostridium diffi cile infection, although we strongly suspect it acts by restoration of microbiotic richness. No major adverse eff ects have yet been reported from FMT, although the keyword in this sentence is “ yet ” . Major adverse eff ects are sure to come, either as short-term complications, likely infectious or allergic, or as long-term unsuspected sequelae, which developed because of alteration in the recipient ’ s intestinal microbiota from his or her own unique community to one closer to that of the donor. We are becoming aware, at an increasingly rapid rate, of the many and varied diseases that have relationships with defi ned microbiota profi les, e.g., C. diffi cile infection ( 2 ), obesity ( 3 ), irritable bowel syndrome ( 4 ), and autism ( 5 ), among others. We are just at the threshold of learning what the precise role our commensal microbiota has in maintaining our health and in causing disease, both gastrointestinal and non-gastrointestinal. Th e importance of the letter to which I am responding is again to alert us that we must constantly be alert, for we certainly do not wish to harm our patients by our ministrations. Once again we are reminded that all treatments, regardless of how innocuous they appear at fi rst, have risks, potential risks, and risks we do yet even appreciate because we know so little.