OBJECTIVE Antibiotic administration has become part of the standard of care for patients with preterm premature rupture of membranes (PROM). Yet, the natural history of intrauterine infection/inflammation during antibiotic therapy remains largely unknown. This study was conducted to determine if antibiotic administration to the mother eradicates intra-amniotic infection and/or reduces the frequency of intra-amniotic inflammation, a risk factor for impending preterm labor/delivery and adverse neonatal outcome. METHODS A subset of patients with preterm PROM admitted to our institution underwent amniocenteses before and after antibiotic administration in order to guide clinical management. Amniotic fluid analysis consisted of a Gram stain, culture for aerobic and anaerobic bacteria as well as genital mycoplasmas, and amniotic fluid white blood cell (WBC) count. Microbial invasion of the amniotic cavity (MIAC) was defined as a positive amniotic fluid culture. Intra-amniotic inflammation was defined as an amniotic fluid WBC count >or=100/mm(3). Patients were given antibiotics and steroids after the 24(th) week of gestation. Antibiotic treatment consisted of ampicillin and erythromycin for 7 days for patients without evidence of intra-amniotic inflammation or MIAC, and ceftriaxone, clindamycin and erythromycin for 10-14 days for those with intra-amniotic inflammation or MIAC. RESULTS Forty-six patients with preterm PROM whose first amniocentesis was performed between 18 and 32 weeks (median 27.4 weeks) were included in the study. The overall prevalence of intra-amniotic inflammation in the first amniocentesis was 39% (18/46). Seven had a positive amniotic fluid culture for microorganisms. At the time of the second amniocentesis, six of the seven patients with a positive amniotic fluid culture had microorganisms. Of 18 patients with intra-amniotic inflammation at admission, only three showed no evidence of inflammation after antibiotic treatment. Among patients with no evidence of intra-amniotic inflammation at admission, 32% (9/28) developed inflammation despite therapy. Five of these nine patients had positive amniotic fluid cultures. CONCLUSIONS (1) Antibiotic administration (ceftriaxone, clindamycin, and erythromycin) rarely eradicates intra-amniotic infection in patients with preterm PROM; (2) intra-amniotic inflammation developed in one-third of patients who did not have inflammation at admission, despite antibiotic administration; (3) a sub-group of patients with documented inflammation of the amniotic cavity demonstrated a decrease in the intensity of the inflammatory process after antibiotic administration.