This study was performed in order to assess whether the use of oral ampicillin vs. placebo therapy in women with latent-phase preterm labor delays delivery and improves pregnancy outcome. A prospective, randomised, double-blind, placebo-controlled study was performed of 110 women at 22–34 weeks of gestation. Patients received oral ampicillin 1 g (n = 59) or placebo (n = 51) every 8 h for 7 days. Outcome measures comprised gestational age at delivery, birth weight, Apgar score, perinatal death, neonatal infection, fetal growth restriction, histological chorioamnionitis and puerperal infection. In the ampicillin and placebo groups, respectively, the average gestational age at birth was 36.6 and 33.8 weeks (p < 0.05) and average birth weight 2,885 and 2,336 g (p 35 weeks and for birth weight ≧2,500g was 3.9 (95% CI 1.6-9.4). Neonatal infection (OR 15.5; 95% CI 4.6–65.7), intrauterine growth retardation (OR 9.6; 95% CI 2.8–41.3), histological chorioamnionitis (OR 13.0; 95% CI 3.4–72.2) and puerperal endometritis-myometritis (OR 65.3; 95% CI 9.3–2,715.7) occurred significantly less frequently among ampicillin-treated than among placebo-treated women. It is concluded that women in latent phase of preterm labor benefit from treatment with oral ampicillin, which prolongs the gestational age at delivery with ensuing increase in birth weight, reduction in the prevalence of histological chorioamnionitis and in neonatal and puerperal infectiou