BACKGROUND: Bacterial vaginosis commonly is found in women with pelvic inflammatory disease (PID), but it is unclear whether bacterial vaginosis leads to incident PID. METHODS: Women (n = 1,179) from 5 U.S. centers were evaluated for a median of 3 years. Every 6-12 months, vaginal swabs were obtained for gram stain and culture of microflora. A vaginal microflora gram stain score of 7-10 was categorized as bacterial vaginosis. Pelvic inflammatory disease was diagnosed by presence of either histologic endometritis or pelvic pain and tenderness plus one of the following: oral temperature greater than 38.3°C; sedimentation rate greater than 15 mm/hour; white blood count greater than 10,000; or lower genital tract detection of leukorrhea, mucopus, or Neisseria gonorrhoeae or Chlamydia trachomatis. RESULTS: After adjustment for relevant demographic and lifestyle factors, baseline bacterial vaginosis was not associated with the development of PID (adjusted hazard ratio 0.89, 95% confidence interval 0.55-1.45). Carriage of bacterial vaginosis in the previous 6 months before a diagnosis (adjusted risk ratio 1.31, 95% confidence interval 0.71-2.42) also was not significantly associated with PID. Similarly, neither absence of hydrogen peroxide-producing Lactobacillus nor high levels of Gardnerella vaginalis significantly increased the risk of PID. Dense growth of pigmented, anaerobic gram-negative rods in the 6 months before diagnosis did significantly increase a woman's risk of PID (P = .04). One subgroup of women, women with 2 or more recent sexual partners, demonstrated associations among bacterial vaginosis, Gardnerella vaginalis, anaerobic gram-negative rods, and PID. CONCLUSION: In this cohort of high-risk women, after adjustment for confounding factors, we found no overall increased risk of developing incident PID among women with bacterial vaginosis. © 2004 by The American College of Obstetricians and Gynecologists.