Background. Chlamydia trachomatis and/or Neisseria gonorrhoeae account for approximately one-third to one-half of pelvic inflammatory disease (PID) cases. Thus, up to 70% of cases have an unknown, nongonococcal/nonchlamydial microbial etiology. Methods. We investigated the associations of N. gonorrhoeae, C. trachomatis, bacterial vaginosis, anaerobic bacteria, facultative bacteria, and lactobacilli with endometritis among 278 women with complete endometrial histology and culture from the PID Evaluation and Clinical Health Study. Results. Women with acute endometritis were less likely to have H2O2-producing Lactobacillus species (odds ratio OR, 0.1; 95% confidence interval CI, 0.01–0.8) and more likely to be infected with C. trachomatis (OR, 16.2; 95% CI, 4.6–56.6), N. gonorrhoeae (OR, 11.6; 95% CI, 4.5–29.9), diphtheroids (OR, 5.0; 95% CI, 2.1–12.2), black-pigmented gram-negative rods (OR, 3.1; 95% CI, 1.4–7.0), and anaerobic gram-positive cocci (OR, 2.1; 95% CI, 1.0–4.3) and to have bacterial vaginosis (OR, 2.4; 95% CI, 1.3–4.3). Conclusions. We conclude that bacterial vaginosis—associated organisms are frequent among women with PID. Because these organisms were strongly associated with endometritis, we recommend that all women with PID be treated with regimens that include metronidazole.