Objectives: We performed a phase I trial to assess the safety and tolerance of a Lactobacillus vaginal suppository for prevention of recurrent UTI. Methods: Premenopausal women with a history of recurrent UTI were randomized to use L. crispatus CTV-05 or placebo vaginal suppositories daily for five days. Results: 30 women were randomized (15 to L. crispatus CTV-05). No severe adverse events occurred. Mild to moderate vaginal discharge and genital irritation were reported by women in both study arms. Seven women randomized to L. crispatus CTV-05 developed pyuria without associated symptoms. Most women had high concentrations of vaginal H202-producing lactobacilli before randomization. L. crispatus, L. jensenii, and L. gasseri were the most common Lactobacillus species identified, with stable prevalence over time. Conclusions: L. crispatus CTV-05 can be given as a vaginal suppository with minimal sideeffects to healthy women with a history of recurrent UTI. Mild inflammation of the urinary tract was noted in some women.