The prevalence of sexually transmitted pathogens in two groups of women was studied: 50 women with clinical diagnoses of acute pelvic inflammatory disease (PID) and 50 asymptomatic women attending a family planning clinic (FPC). Genital specimens, collected by non-invasive procedures, were examined. Endocervical Neisseria gonorrhoeae was present in 62% of the PID group and 10% of the FPC group (P less than 0.0001). One-third of N. gonorrhoeae isolates were penicillinase-producing strains. Chlamydia trachomatis was isolated from the endocervix in 30% of the PID group and 26% of the FPC group (P = 0.8240 NS). Mycoplasma hominis was more prevalent in the vaginas and endocervices of the PID group than the FPC group (84% and 72% v. 50% and 42%; P = 0.0006 and 0.0047 respectively). Trichomonas vaginalis was present in 56% of the PID group and 20% of the FPC group (P = 0.0004). Syphilis serology was positive in 34% of the PID group and 10% of the FPC group (P = 0.0026). In the PID group of patients, 8% were positive for HBsAg. Antibody to the human immunodeficiency virus was not detected in any of the 100 women. The high prevalence of recognised sexually transmitted pathogens underlines the need for appropriate antimicrobial agent(s) active against N. gonorrhoeae, C. trachomatis and M. hominis in patients with PID. In view of the high prevalence of penicillinase-producing strains of N. gonorrhoeae, routine use of an antibiotic active against such strains is desirable.