OBJECTIVE: Our purpose was to evaluate the association of bacterial vaginosis, trichomonas vaginitis, and monilial vaginitis with spontaneous preterm birth at<35 weeks 0 days. STUDY DESIGN: A total of 2929 women at 10 centers were studied at 24 and 28 weeks' gestation by Gram stain of vaginal smear, wet mount, and 10% potassium hydroxide preparations to detect vaginal infections. RESULTS: The rates of detected infection at 24 and 28 weeks, respectively, were bacterial vaginosis 23.4% and 19.4%, trichomonas 3.3% and 2.7%, and monilia 21.1% and 19.5%. The occurrence of bacterial vaginosis at 28 weeks was associated with an increased risk of spontaneous preterm birth, odds ratio 1.84 (95% confidence interval 1.15 to 2.95, p < 0.01). Detection of Trichomonas vaginalis (by wet mount) or monilia (by potassium hydroxide preparation) had no singificant associations with preterm birth. CONCLUSION: The presence of bacterial vaginosis at 28 weeks' gestation is associated with an increased risk of spontaneous preterm birth. © 1995.