Controversy surrounds the association between bacterial vaginosis (BV) and pelvic inflammatory disease (PID). Women (N = 1,140) were ascertained at five US centers, enrolled (1999-2001), and followed up for a median of 3 years. Serial vaginal swabs were obtained for Gram's stain and cultures. PID was defined as 1) histologic endometritis or 2) pelvic pain and tenderness plus oral temperature >38.8°C, leukorrhea or mucopus, erythrocyte sedimentation rate >15 mm/hour, white blood cell count >10,000, or gonococcal/chlamydial lower genital infection. Exploratory factor analysis identified two discrete clusters of genital microorganisms. The first correlated with BV by Gram's stain and consisted of the absence of hydrogen peroxide-producing lactobacillus, Gardnerella vaginalis, Mycoplasma hominis, anaerobic Gram-negative rods, and, to a lesser degree, Ureaplasma urealyticum. The second, unrelated to BV by Gram's stain, consisted of Enterococcus species and Escherichia coli. Being in the highest tertile in terms of growth of BV-associated microorganisms increased PID risk (adjusted rate ratio = 2.03, 95% confidence interval: 1.16, 3.53). Carriage of non-BV-associated microorganisms did not increase PID risk. Women with heavy growth of BV-associated microorganisms and a new sexual partner appeared to be at particularly high risk (adjusted rate ratio = 8.77, 95% confidence interval: 1.11, 69.2). When identified by microbial culture, a combination of BV-related microorganisms significantly elevated the risk of acquiring PID. Copyright © 2005 by the Johns Hopkins Bloomberg School of Public Health. All rights reserved.