The hypothesis is advanced that the transition from a Lactobacillus-dominated vaginal microflora to a microflora characteristic of bacterial vaginosis (BV), as well as development of the adverse consequences of BV in some women but not in others, are due to alterations in innate immunity. A microbial-induced inhibition of Toll-like receptor expression and/or activity may block induction of proinflammatory immunity and lead to the proliferation of atypical vaginal bacteria. A lack of 70-kDa heat-shock protein production and release in response to abnormal flora would compound this failure to activate antimicrobial immune responses. A deficit in vaginal mannose-binding lectin concentrations would further decrease the capacity for microbial killing and increase the likelihood of bacterial migration from the vagina to the upper genital tract.