Preterm delivery (PTD) is the leading cause of infant morbidity and mortality in the United States. An epidemiological association between PTD and various bacteria that are part of the vaginal microflora has been reported. No single bacterial species has been identified as being causally associated with PTD, suggesting a multifactorial etiology. Quantitative microbiologic cultures have been used previously to define normal vaginal microflora in a predictive model. These techniques have been applied to vaginal swab cultures from pregnant women in an effort to develop predictive microbiologic models for PTD. Logistic regression analysis with microbiologic information was performed for various risk groups, and the probability of a PTD was calculated for each subject. Four predictive models were generated by using the quantitative microbiologic data. The area under the curve (AUC) for the receiver operating curves ranged from 0.74 to 0.94, with confidence intervals (CI) ranging from 0.62 to 1. The model for the previous PTD risk group with the highest percentage of PTDs had an AUC of 0.91 (CI, 0.79 to 1). It may be possible to predict PTD by using microbiologic risk factors measured once the gestation period has reached the 20-week time point.