Objective To evaluate the efficacy of additional topical Lactobacillus casei rhamnosus (Lcr35) subsequent to antibiotic treatment of bacterial vaginosis (BV) to restore the normal vaginal flora. Study design Single-centre, randomised, observerblinded study. Setting Population-based study in Vienna over 1 year. Sample 190 women were enrolled in the study. Methods Women with Nugent scores between 7 and 10 on initial vaginal swab were randomised to the one of two groups. All women were treated with standard antibiotic therapy for 7 days. Only women in the intervention group received vaginal capsules containing 10(9) colony-forming units of live Lcr35 for 7 days after antibiotic treatment. Final vaginal swabs for Nugent scoring were taken 4 weeks after the last administration of the study medication. Main outcome measures The primary efficacy variable was a change in the Nugent score between the baseline and the end of the study of at least 5 grades in each individual woman. Results Sixty-nine of the 83 women (83%) in the intervention group and 31 of the 88 women (35%) in the control group showed a reduction of the Nugent score by at least 5 grades. The difference in the number of women with improvement was highly significant (P < 0.001). The median difference in Nugent scores between initial and final swabs was 6.61 in the intervention group and 4.13 in the control group (P < 0.001). Conclusion Our data show that the restoration of the vaginal flora after antibiotic treatment of BV can be significantly enhanced by exogenously applied lactobacilli.