We review the impact of antimicrobial treatment on maternal and fetal outcome during expectant management of preterm premature rupture of the membranes. Relevant studies were retrieved from Medline (1966 to August, 1994) with the search term fetal-membrane-premature-rupture and antibiotics or antimicrobial, Excerpta Medica (1972 to August, 1994) with the search term premature fetus, membrane rupture, and antibiotic or antimicrobial therapy, and the Cochrane database of systemic reviews with the criterion antibiotics and prelabour rupture of membranes. We also obtained unpublished data from a randomised clinical trial of ceftizoxime versus placebo. The selected studies were randomised controlled trials of systemic antimicrobial therapy for prolongation of gestation in non-labouring women after preterm premature rupture of the membranes. Data extraction was done by a single reviewer. Studies were evaluated for post-randomisation exclusion and other confounding variables that might introduce analytical bias. Analysis was done with SAS statistical software by a blinded investigator. Antimicrobial therapy after preterm premature rupture of the membranes is associated with a reduced number of women delivering within 1 week (62 vs 76%; OR 0.51, 95% CI 0.41-0.68), and reduced diagnosis of maternal morbidity including chorioamnionitis (12 vs 23%; 0.45, 0.33-0.60) and postpartum infection (8 vs 12%; 0.63, 0.41-0.97). Fetal morbidity, including confirmed sepsis (5 vs 9%; 0.57, 0.36-0.88), pneumonia (1 vs 3%; 0.32, 0.11-0.96), and intraventricular haemorrhage (9 vs 14%; 0.65, 0.45-0.92) were less often diagnosed after antimicrobial therapy. Separate analysis of the six placebo-controlled trials revealed similar or improved odds of pregnancy prolongation, chorioamnionitis, neonatal sepsis, postpartum infection, positive infant blood cultures, and pneumonia. Antimicrobial therapy, when used in the expectant management of preterm premature rupture of the membranes is associated with prolongation of pregnancy and a reduction in the diagnosis of maternal and infant morbidity. Further study should be directed towards determination of optimal antimicrobial therapy, increasing pregnancy prolongation, and enhancement of corticosteroid therapy for induction of pulmonary maturity after preterm premature rupture of the membranes.