OBJECTIVES: To prospectively assess the efficacy and safety of Lactobacillus vaginal suppositories for the prevention of recurrent cystitis. METHODS: In this single-arm, open-label, phase II clinical trial, participants used vaginal suppositories containing the GAI 98322 strain of Lactobacillus crispatus for 1 year either every 2 days or three times per week. The primary end-point was the response rate, as assessed by the number of episodes of recurrent cystitis during the year of administration. The secondary end-points were the response rate, as assessed by episodes of recurrent cystitis during the 1 year after completion of the administration period; the total number of episodes of recurrent cystitis before, during and after administration; adverse events; and changes in urine bacteria and the vaginal microbiome. RESULTS: A total of 28 women were enrolled, and 21 completed the study. A total of 18 patients achieved an effective response (86%) during administration. The suppressive effects of Lactobacillus vaginal suppositories on episodes of cystitis continued up to 1 year after the last suppository was administered. There was a significant reduction in the mean number of episodes of cystitis, both during and after administration of Lactobacillus vaginal suppositories. No treatment-related adverse events were observed. Amplicon sequencing analysis of the vaginal microbiome showed that Lactobacillus species colonized the vagina during the periods when episodes of cystitis were absent. CONCLUSIONS: Vaginal suppositories containing the GAI 98322 strain of Lactobacillus crispatus effectively prevent episodes of recurrent cystitis, both during administration and for at least 1 year after administration.