The efficacy and safety of two antibiotic regimens for the treatment of acute pelvic inflammatory disease (PID) was compared in a prospective and randomised study. 57 patients received either 0.2 gms ciprofloxacin intravenously b.i.d. in combination with 0.5 g metronidazole intravenously t.i.d. (n = 26), or alternatively 2 g cefoxitin intravenously t.i.d. in combination with doxycycline 0.1 g b.i.d. (n = 31). After commencing therapy intravenously, medication with ciprofloxacin, metronidazole and doxycycline was continued orally after two or three days. In the ciprofloxacin/metronidazole group, PID was found to be severe in 7, moderate in 12 and mild in 7 patients. The numbers in the cefoxitin/doxycycline group were 8, 20 and 3 respectively. The clinical result after treatment with ciprofloxacin/metronidazole was resolution of all symptoms in 24 patients and improvement in 2 others. In the cefoxitin/doxycycline treated group, resolution was found in 27 patients, improvement in 2 others. Failure occurred in 2 patients. 53 different microorganisms as the suspected cause of PID were isolated in the ciprofloxacin/metronidazole group and 56 in the cefoxitin/doxycycline group. According to our clinical and bacteriological criteria, treatment for PID was successful in 97% of the ciprofloxacin/metronidazole group and in 87% of the cefoxitin/doxycycline group. Adverse reactions were found in 4 patients in the ciprofloxacin/metronidazole treated group. Therapy had to be terminated in 3 of these patients. In the cefoxitin/doxycycline group 2 patients had adverse reactions, and therapy had to be terminated in one of these patients. According to our results, both antibiotic regimens can be recommended for the treatment of PID.