OBJECTIVE: The purpose of this study was to evaluate the evidence regarding antibiotics for the treatment of preterm labor. STUDY DESIGN: Through dual review, we abstracted study design and masking, definitions of preterm labor and pregnancy outcome, patient inclusion/exclusion characteristics, patient demographic characteristics, drug and cointerventions, and numerous birth, maternal, and neonatal outcome measures. We graded the quality of the individual articles and the strength of the evidence for antibiotic benefit. RESULTS: We abstracted data from 14 randomized trials and 1 observational study. Of these studies, 13 trials met the requirements for a meta-analysis. The meta-analysis demonstrated a mixed outcome pattern with small improvements in pregnancy prolongation, estimated gestational age at birth, and birth weight. Data were insufficient to show a beneficial effect on neonatal morbidity or mortality rates. CONCLUSION: Treatment of preterm labor with antibiotic therapy can prolong gestation. The benefits of antibiotics are small, and there is considerable uncertainty about the optimal agent, route, dosage, and duration of therapy. (Am J Obstet Gynecol 2002;186:587-92.)