Our purpose was to compare the efficacy of 25 lug and 50 mug intravaginally administered misoprostol tablets for cervical ripening and labor induction. Either 25-mug (n: 58) or 50-mug (n: 56) misoprostol tablets were randomly administered intravaginally to 114 subjects with an unripe cervix for labor induction. The physician was blinded to the medication. Intravaginal misoprostol was given every 4 h until the onset of labor. The mean Bishop score before misoprostol administration was 2.1 +/- 1.6 in the 25-mug group and 2.0 +/- 1.4 in the 50-mug group (p > 0.05). With the 25-mug dose the time until delivery was significantly longer (991.2 +/- 514.4 min vs. 703.12 +/- 432.6 min in the 50-mug group). The use of oxytocin augmentation was significantly higher in the 25-mug group (63.8%) than the 50-mug group (32.1%; p < 0.05). The proportions of patients with tachysystoles and hypersystoles were not significantly different between the two groups (19 and 6.9%, respectively, in the 25-mug group and 25 and 17.8%, respectively, in 50-mug group; p > 0.05). Overall, in the 25mug group more women achieved vaginal delivery (79.3 vs. 60.7%; p < 0.05). The rate of cesarean sections due to nonreassuring fetal status was higher in the 50-mug misoprostol group (28.6 vs. 10.3%; p < 0.05). The number of neonates with a low 1-min Apgar score (<7) was significantly higher in the 50-mug misoprostol group (26.8 vs. 8.6%; p < 0.05), but 5-min Apgar scores and umbilical artery blood gas values at the time of delivery were not significantly different between the groups (p > 0.05). One patient in the 25-mug group suffered a ruptured uterus. Intravaginal administration of 25 lug of misoprostol is a clinically effective labor induction regimen and has the least adverse effects and complications. Copyright (C) 2002 S. Karger AG, Basel.