A prospective, randomized, controlled clinical trial was conducted to evaluate the safety and efficacy of a single, 0.5-mg dose of prostaglandin E2 gel in a shelf-stable triacetin base administered intracervically in patients with an unfavorable cervix (Bishop score less than or equal to 4) 12 hours prior to oxytocin-induction of labor. forty-eight evaluable patients were enrolled, 25 in the PGE2 group and 23 in the control group. PGE2 gel treatment resulted in a significant improvement in the mean Bishop score compared to control (3.14 versus 0.70, P less than .00005). Sixty-four percent of the patients treated with PGE2 had regular uterine contractions during the 12-hour preinduction period beginning 2.18 +/- 2.0 hours after gel insertion, compared to 9% in the control group, P = .0001. Moreover, 12% (3/25) of the patients receiving PGE2 progressed into active labor and delivered during the preinduction period. The duration of oxytocin infusion required for the induction or augmentation of labor was significantly shorter for patients who received intracervical PGE2 gel compared to the control group (13.1 +/- 8.1 versus 19.0 +/- 8.7, P less than .05). However, there was no significant difference in the incidence of cesarean section between the two study groups (40 and 22% in the PGE2 and control groups, respectively). No episodes of uterine hypertonus or hyperstimulation or gastrointestinal side effects occurred as a result of PGE2 gel treatment.(ABSTRACT TRUNCATED AT 250 WORDS)