Objective: To investigate the prevalence of abnormal vaginal flora (AVF) and predictive factors for intrauterine infection in pregnant Korean women with preterm labor. Materials and Methods: The authors reviewed the medical records of 106 pregnant Korean women with preterm labor admitted to Eulji Medical Center between January 2006 and August 2011. The results of vaginal discharge tests and maternal serum C-reactive protein (CRP) level at admission, placental biopsy, and perinatal outcomes were searched. The prevalence of abnormal vaginal flora was calculated. The perinatal outcomes and predictive factors for intrauterine infections were analyzed based on placental pathology and early-onset neonatal sepsis. Results: The prevalence of abnormal vaginal flora was 75.4%. Ureaplasma urealyticum (UU), intermediate flora, Candidiasis, bacterial vaginosis, and aerobic bacterial colonization were detected in 40.6%, 38.7%, 17%, 14.2%, and 11.3% of the women, respectively. The frequency of early-onset neonatal sepsis was significantly different between women with aerobic bacterial colonization and those with normal flora (p = 0.008). An elevated maternal serum CRP level was an independent intrauterine infection predictor (odds ratio, 1.918; 95% confidence interval, 1.102-3.338; p = 0.048). Conclusion: Aerobic bacterial colonization may predict early-onset neonatal sepsis. An elevated maternal serum CRP level was an independent intrauterine infection predictor based on placental infections and early-onset neonatal sepsis.