Healthy female volunteers participated in an anonymous study to monitor vaginal flora by taking daily vaginal samples and making a smear for later Gram-staining, as well as recording information on genital symptoms, sexual activity, contraceptive and bathing practices. A modification of Spiegel's criteria was used to categorize the Gram-stained smears, an intermediate category between normal flora and bacterial vaginosis (BV) being recognized. Of the 22 volunteers who completed the study, one was excluded because of pregnancy. Of the remaining 21 women, 10 (48%) had a normal flora throughout the study, 4 (19%) had an abnormal flora throughout and 7 (33%) had a basically normal flora which underwent a change to either an intermediate flora in 5 women or fully developed BV in 2 of them. In 5 (71%) of these women the change occurred within the first 9 days of the cycle. Transient changes in the vaginal microbial flora occurred predominantly in the first part of the menstrual cycle which suggests that in some women hormonal changes could have a role in the pathogenesis of bacterial vaginosis.