Background: Diarrhea is a common adverse event of tyrosine kinase inhibitors (TKI). No standard treatment for TKI-induced diarrhea has been established, although probiotics are commonly used. In several patients TKI dose reduction is necessary. Data showing the interplay between microbiota and TKI-induced diarrhea are reported. The therapeutic modulation of gut microbiota could be useful to ameliorate TKI-related diarrhea. Our study aims to explore the efficacy and safety of fecal microbiota transplantation (FMT), compared with current clinical practice approach, for the treatment of TKI-associated diarrhea in patients with metastatic renal cell carcinoma. Methods: Patients on treatment with Pazopanib/Sunitinib as first line therapy for metastatic renal cell carcinoma and TKI-associated diarrhea (³ 4 stools per day over baseline) were randomize to receive FMT from healthy donor by colonoscopy (fresh faeces) or probiotics (L. casei DG). The primary endpoint was the resolution of TKI-related diarrhea. Baseline fecal samples were collected from all patients and donors. All patients were followed up 7, 15, 30 and 60 days after the treatment for diarrhea. Results: 21 subjects, 10 in FMT arm and 11 in control arm, have been enrolled. At 7 day-follow-up, TKI-related diarrhea disappeared in 10 patients of the FMT group and in 6 patients of the control group (100% vs 54.5%, p = 0.02). At 15-day and 30-day, 9 patients in the FMT group and 0 patients in the control group experienced resolution of diarrhea (90% vs 0%, p = 0.0001). At 60-days 8/10 patients in FMT arm did not have diarrhea. Dose reduction was necessary in 4 patients of the control group. No serious adverse events associated with any of the two treatment protocols were observed. Metagenomic analyses on collected stool samples are ongoing. Conclusions: This open-label randomised controlled trial showed the effectiveness and safety of FMT compared with probiotics in curing TKI-related diarrhea in patients with metastatic renal cell carcinoma, avoiding the necessity of dose reduction. Considering the increasing evidence, the modulation of microbiota trough FMT could also affect the efficacy of immunotherapy for metastatic renal cell carcinoma.