We studied 204 women with acute pelvic inflammatory disease to delineate further the causes of that illness. Gonococci were recovered from 91. Gonococcal pili antibody rose or fell significantly in 12 of 18 patients with positive cultures and only two of 19 who had negative cultures and smears for Neisseria gonorrhaoea(P smaller than 0.005). N. gonorrhoeae was found in peritoneal exudate from eight of 21 patients with, and none of 33 without, cervical gonococcal infection. Among patients with severe disease, other bacteria were recovered from peritoneal exudates from five of 16 with, and 19 of 22 without, cervical gonococcal infection (P smaller than 0.025). Mixed anaerobic and aerobic bacterial peritoneal infection was common in nongonococcal pelvic disease. The most common species recovered were Bacteroides fragilis, peptostreptococci, and peptococci. Tuboperitoneal gonococcal infection probably causes pelvic inflammatory disease in most patients with cervical gonococcal infection, whereas polymicrobial tuboperitoneal infection probably causes most nongonococcal cases.