The aim of this study is the evaluation of predictive factors in the onset of labour after pre-induction cervical ripening with prostaglandins. We enrolled 112 consecutive singleton term pregnancies (37-42.3 weeks) with unfavourable cervix and intact membranes, requiring induction of labour because prolonged pregnancy (59%) or maternal/fetal complications (41%). Treatment consisted of the cervical application (once or twice, 12 h apart) of prostaglandin E2 gel (Upjohn, Italy). Uterine activity was monitored by external cardio-tocography before and during the next 2 h. Two patients showed uterine hyperstimulation and acute fetal distress requiring caesarean section. Sixty percent of patients went to labour and delivered without further stimulations. In this group the rate of caesarean section (9.1%) was lower than in patients failing to onset labour (68.2%). According to the logistic regression three factors positively predicted the onset of labour: first-hour uterine contractility, basal uterine activity and gestational age. The first-hour contractility in particular, represents the myometrial sensitivity to prostaglandin E2 and may become a practical marker of spontaneous onset of labour in patients undergoing cervical ripening.