Key messages c Bacterial vaginosis (BV) prevalence was extremely high in this cohort of HSV-2 positive women. c Consistent with studies in industrialised countries and Africa, we found strong independent associations of BV with HIV, Trichomonas vaginalis and recent sex. c Vaginal cleansing was very common in this cohort, but we found no association between BV and the frequency, timing or methods of cleansing. c BV prevalence was significantly lower among women who reported inserting vaginal drying substances, although that practice was relatively uncommon. seroconversion than those without BV.8 However, a trial of presumptive STI treatment to reduce HIV acquisition showed a marked decrease in BV without any impact on HIV,29 and HIV infection may be associated with increased prevalence and persistence of BV.24 There are several limitations to our analysis. This is a cross- sectional analysis and so we cannot determine causality of the observed associations. Residual confounding may have remained when adjusting for self-reported risk factors, especially sexual behaviours, which are difficult to measure accurately. The risk factors for BV that we identified may not be generalisable since the women in our study were at high risk of other RTIs and all were HSV-2 positive. Although women were asked to avoid vaginal cleansing before they attended the clinic many reported cleansing in the past 24 hours, which possibly reduced the accuracy of our diagnosis of BV. However, we found no association between women’s reported time of last cleansing and BV. Strengths of our study include a large sample size and the use of an expert microbiology laboratory with external quality control to diagnose BV. Given the association of BV with HIV acquisition shown in many studies, and the extremely high prevalence of BV in this and other African populations, a high proportion of HIV infections may be attributable to BV. Further research is warranted on preventable risk factors for BV, including specific vaginal practices that may alter the vaginal flora. Acknowledgements: We would like to thank the Wellcome Trust, the UK Medical Research Council and the Department for International Development UK for funding the study; Mary Rusizoka, Tim Clayton and the HSV trial team; Mary Louise Shushu, and the laboratory and data staff of the Mwanza National Institute for Medical Research; the administrative staff of AMREF Mwanza; Gerald Baldrey and Geita Gold Mine; Tamara Hurst and Fiona Marquet of the Wellcome Trust Bloomsbury Centre for Clinical Tropical Medicine and Lucy Bradshaw at LSHTM. We are very grateful to the Tanzanian Ministry of Health for permission to conduct and publish this study. We gratefully acknowledge the support of the trial steering committee (A Nunn, F Cowan, S Kapiga, H Mshinda) and the Data Monitoring Committee (D Dunn, H Grosskurth, W Kilama). Funding: Funding for the study was provided by the Wellcome Trust, the UK Medical Research Council (MRC) and the UK Department for International Development. Competing interests: None. Ethics approval: The study protocol was approved by the Medical Research Coordinating Committee of Tanzania and the London School of Hygiene and Tropical Medicine ethics committee (ISRCTN 35385041). Contributors: KB analysed the data and wrote the first draft of the manuscript; DWJ was principal investigator and supervised the trial; DWJ, RH and HW designed the study; KB, IH and TC supervised the data management; JC, DE and PH supervised the laboratory work; PH performed the external QC; CT and KM supervised fieldwork; all authors commented on drafts of the manuscript and approved the final version. E p i d e m i o l o g y REFERENCES 1. Svare JA, Schmidt H, Hansen BB, et al. 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