Alterations in the lower genital tract flora were studied during antimicrobial treatment of 36 women with acute pelvic inflammatory disease (PID). Sixteen women received ciprofloxacin, and 20 received doxycycline plus metronidazole. Both regimes eradicatedChlamydia trachomatis in all cases andNeisseria gonorrhoeae in all except one in which combination treatment failed because of resistance of the organism to tetracycline. Both regimes eradicatedMycoplasma hominis in all cases, while ciprofloxacin was somewhat less effective than combination treatment againstUreaplasma urealyticum. Ciprofloxacin was less effective than combination treatment againstGardnerella vaginalis. The incidence ofCandida infection increased equally in both groups, although symptoms necessitating antifungal treatment were rare.