To define and quantitate histologic changes in the endometrium that best correlate with documented upper genital tract infection (UGTI) and laparoscopically diagnosed acute salpingitis, we studied endometrial biopsy specimens from 69 consecutive patients with clinically suspected acute pelvic inflammatory disease (PID) who underwent microbiological evaluation for UGTI and laparoscopic examination for acute salpingitis. Both UGTI and acute laparoscopically confirmed salpingitis were present in 37 patients (54%), UGTI without salpingitis in 1 (1%), salpingitis without UGTI in 11 (16%), and neither UGTI nor salpingitis in 20 (29%). Chlamydia trachomatis or Neisseria gonorrhoeae UGTI was found in 34 women, Escherichia coli in two patients, Peptococcus magnus in one woman, and with Streptococcus agalactiae in one woman. The following features were correlated both with UGTI and with salpingitis: presence of any neutrophils in the endometrial surface epithelium; neutrophils within gland lumens; dense subepithelial stromal lymphocytic infiltration; any stromal plasma cells; and germinal centers containing transformed lymphocytes. The simultaneous presence of five or more neutrophils per X 400 field in endometrial surface epithelium, together with one or more plasma cell per X 120 field in endometrial stroma, was the best predictor of UGTI plus salpingitis. This combination had a sensitivity of 92% and a specificity of 87% for predicting the diagnosis of both UGTI and laparoscopically confirmable acute salpingitis. Prospective studies are needed to assess the usefulness of these criteria.