Hormonal changes may be important in the onset and clearance of bacterial vaginosis. We studied vaginal flora and serum oestradiol levels of 55 women at baseline and during hormonal treatment. None developed bacterial vaginosis (BV) from normal vaginal flora, 69% of women had normal flora at baseline increasing to 91% following hormonal treatment. The mean oestradiol level with BV was 39.07 ng/L compared with 176.41 ng/L with normal flora. Non-smokers had a mean oestradiol level of 173.95 ng/L compared with 118.67 ng/L in smokers. Recombinant follicle-stimulating hormone resulted in a mean oestradiol rise of 113.9 ng/L. The mean rise was 330.4 ng/L with improved vaginal flora but only 15.1 ng/L in persistently abnormal or worsening flora. A rise in oestradiol in this group of women was associated with a significant reduction of abnormal flora. Reversion from BV to normal flora was associated with a greater rise in oestradiol than where abnormal flora persisted or worsened. This study supports a possible hormonal influence in the natural history of BV. The lower oestradiol levels in smokers may help explain their increased risk of BV.