To study the bacterial pathogenesis of acute pelvic inflammatory disease, peritoneal fluid was obtained by culdocentesis in 133 of 344 women with this disease. In 104 of the specimens bacteria were identified both in the gram-stained smear and culture. Neisseria gonorrhoeae was isolated from the lower genital tract in over half of these women, and there were 3 patterns of bacterial recovery from peritoneal fluid: N gonorrhoeae alone (22%), N gonorrhoeae and other organisms (32%), and nongonococcal organisms alone (46%). In women without cervical gonorrhea only nongonococcal organisms were identified from peritoneal fluid. In both groups of women a similar number of nongonococcal organisms were isolated. The results of this study supported those reported prior to availability of antimicrobials and suggest that A' gonorrhoeae initiates most cases of pelvic inflammatory disease. A significant number of these women have superinfection with nongonococcal organisms which may preclude recovery of gonococci. © 1978 The American College of Obstetricians and Gynecologists.