Objective To compare wet smear and Gram stain diagnoses of bacterial vaginosis among asymptomatic pregnant women.Methods Between September 1,2006 and January 31,2008,asymptomatic women who initiated prenatal care in our obstetric clinics were invited to participate.Clinical diagnosis that required two of three positive criteria for bacterial vaginosis(vaginal pH,whiff test,and clue cells on wet smear)was compared with Gram stain diagnosis(Bacterial vaginosis score 7-10 by Nugent criteria).Results Population characterstics included an average matemal age of(27.3±6.6)years,26 nulliparas.The mean gestational age at entry was(15.6±7.6)weeks.Twenty-seven percent(18/67)of the study population was diagnosed with bacterial vaginosis by definitive Gram stain.Using Gram stain diagnosis of bacterial vaginosis as the standard,clinical diagnosis had sensitivity of 56%(95% confidence interval[CI]32%,78%),aspecificity of 96%(95% contidence interval[CI]90%,100%),a positive predictive value of 83%,and a negative predictive value of 85%.Conclusion In asymptomatic pregnant women,bacterial vaginosis can be diagnosed reliably by Gram stain.