The diagnostic criteria of bacterial vaginosis (BV) and the prevalence of Mobiluncus spp as detected by monoclonal antibodies were investigated in all new women patients attending the sexually transmitted disease (STD) clinic in Uppsala during a four month period. Of 455 patients, 164 fulfilled the generally accepted criteria for BV, but in 57 of them simultaneous infection with a recognised pathogen was diagnosed. BV was thus the only clinical diagnosis in 107 (24%) of the women. The sniff test and clue cells in the wet smear were the two criteria most relevant for the diagnosis of BV. The sniff test was positive in 95% (156) of the 164 patients with BV and negative in all other cases. The corresponding figure for the clue cells was 98% (160 of 164), but clue cells were also detected in 19 patients without BV. Though 99% (162) of women with BV had a vaginal pH of more than 4.5, so did 83 women without BV. Only 59% (96) of women fulfilling the criteria of BV had a characteristic discharge. Mobiluncus spp were present in 20% (90) of the 455 women and in 50% (53) of the 107 women with BV only. Of the 90 Mobiluncus spp isolates, M curtisii comprised 44% (40), M mulieris 34% (31), and both strains together 21% (19). Mobiluncus spp were detected with monoclonal antibodies in 35 women who had no motile curved rods on wet smear microscopy. Furthermore, Mobiluncus spp were often detected in women infected with recognised pathogens, as well as in a few women without signs of genital infection.