Purpose of investigation: To compare the effects of 50 μg of vaginal misoprostol with 25 μg for labor induction at term. Methods: One hundred and forty-seven pregnant women with indications for labor induction and cervical Bishop's score of ≤ 6 were randomly assigned to receive either 50 μg (n = 74) or 25 μg (n = 73)of vaginal misoprostol every four hours until either a Bishop's score of ≥ 8 or adequate uterine contraction frequency had been achieved. Induction-to-vaginal-delivery time was considered the primary outcome measure. Results: Mean induction-to-vaginal-delivery time was significantly shorter in the 50-μg group than in the 25-μg group (526 +/- 141 min vs 745 +/- 218 min, respectively); oxytocin was administered to 65.8% of the patients in the 25-μg group and to 35.1% in the 50-μg group (p < 0.05). The incidence of tachysystole was significantly higher in the 50-μg group than in the 25-μg group (12% vs 2.7%, p < 0.05). We found no statistically significant difference between the two groups with respect to the rate of primary cesarean section, incidence of hyperstimulation syndrome, or neonatal outcome (p > 0.05). Conclusion: Fifty micrograms of vaginally administered misoprostol is an effective and inexpensive means of inducing labor at term. Uterine tachysystole may be associated more frequently with a 50-μg dose of vaginal misoprostol than with a 25-μg dose. Clinicians must accurately document the frequency and intensity of uterine contractions before every 50-μg dose of misoprostol is administered.