To compare the safety and efficacy of two different regimens of misoprostol for labor induction at term, we conducted a randomized controlled trial on women presenting for induction of labor at > or =37 weeks' gestation. Eligible women were randomized to receive intravaginal misoprostol 50 microg every 4 h or 100 microg every 6 h until any of the following: 1) adequate contraction pattern (3 contractions/10 min); 2) dilatation >3 cm; 3) artificial rupture of membranes; or 4) signs of uterine hyperstimulation. Use of oxytocin during labor was at the discretion of the managing clinician. The main outcome variable considered for analysis was cesarean section rate. Secondary outcome measures were induction to delivery interval and neonatal outcome (Apgar scores, meconium staining, and umbilical artery pH). A total of 58 women were randomized to receive either misoprostol 100 microg (n=26) or 50 microg (n=32). The 100 and 50 microg groups had similar mean Bishop's scores at induction (4.0+/-2.3 vs 4.1+/-2.2, p=0.87), rates of nulliparity, use of epidural anesthesia, and oxytocin augmentation. The number of doses of misoprostol used was similar in the two groups (1.4+/-0.6 vs 1.8+/-1.2). The mean+/-standard deviation time to delivery (hours) (11.9+/-7.3 vs 14.3+/-9.6 h, p=0.30) and cesarean section rate (35% vs 19%, p=0.30, relative risk: 1.8, 95% confidence interval 0.7-5.4) were not different in the 100 vs 50 microg group. Power analysis demonstrated that 132 women would be required in each group to achieve statistical significance in the primary outcome measure (alpha=0.05, beta=0.80). Similarly, rates of 5-minute Apgar scores <7 (4% vs 3%, p=1.0), and of meconium passage (17% vs 25%, p=0.73) were not significantly different between the two groups.