BACKGROUND Although not yet registered for such use, misoprostol has been widely used for obstetric and gynaecological indications, such as induction of abortion and of labour. OBJECTIVE The objective of this review was to assess the effects of vaginal misoprostol for third trimester cervical ripening or induction of labour. METHODS The Cochrane Pregnancy and Childbirth Group trials register, the Cochrane Controlled Trials Register and reference lists of articles were searched. METHODS Randomised trials comparing vaginal misoprostol with other methods of cervical ripening or labour induction, placebo or no treatment in women due for third trimester induction of labour. METHODS Trial quality assessment and data extraction were done by both reviewers. RESULTS Twenty-six studies were included. Compared to placebo, misoprostol was associated with increased cervical ripening (relative risk of unfavourable or unchanged cervix after 12 to 24 hours with misoprostol 0.20, 95% confidence interval 0.07 to 0.61). It was also associated with a reduced need for oxytocin (relative risk 0.47, 95% confidence interval 0.23 to 0.96). Misoprostol was more effective than prostaglandin E2 for labour induction (relative risk of failure to achieve vaginal delivery in 24 hours 0.70, 95% confidence interval 0.62 to 0.79). Oxytocin augmentation was used less often with misoprostol than with prostaglandin E2 (relative risk 0.64, 95% confidence interval 0.58 to 0.71). Uterine hyperstimulation and meconium stained liquor were more common with misoprostol than with prostaglandin E2. Lower doses of misoprostol compared to higher doses did not show significant differences except for more need for oxytocin augmentation and less uterine hyperstimulation without fetal heart rate changes. CONCLUSIONS Vaginal misoprostol appears to be more effective in inducing labour than conventional methods of cervical ripening and labour induction. The apparent increase in uterine hyperstimulation is of concern. The studies were not large enough to exclude the possibility of rare but serious adverse effects.