Background. Anal HPV infection, anal dysplasia and, ultimately, anal cancer are particularly common in HIV-infected men who have sex with men. Treatment of anal dysplasia, aiming to prevent evolution to squamous cell carcinoma of the anus, is currently limited to direct ablation and/or application of topical therapy. The aim of the present study is to investigate the effect of oral bacteriotherapy (Vivomixx® in EU, Visbiome® in USA) on anal HPV infection and HPV-related dysplasia of the anal canal in HIV-infected men who have sex with men. Methods. In this randomized, placebo-controlled, quadruple-blinded trial (NCT04099433), HIV-positive men who have sex with men with anal HPV infection and HPV-related dysplasia were randomized to receive oral bacteriotherapy or placebo for 6 months. Anal HPV test, anal cytology and high resolution anoscopy with biopsies of anal lesions were performed at baseline and at the end of the study. Safety and tolerability of oral bacteriotherapy were also evaluated. Interim analysis results were presented. Results. 20 participants concluded the study procedures to date. No serious adverse events were reported. In respect to participants randomized to placebo, individuals in the experimental arm showed higher rate of anal dysplasia regression (p = 0.002), lower rate of onset of new anal dysplasia (p = 0.023) and lower rates of worsening of persistent lesions (p = 0.004). Clearance of anal HPV infection was more frequently observed in the bacteriotherapy group (p = 0.067). Conclusion. Being an interim analysis, we limit ourselves to report the preliminary results of the current study. We refer the conclusions relating to the possible effectiveness of the intervention to the analysis of the definitive data.