ABSTRACTBACKGROUND AND OBJECTIVE: Labor induction in the presence of an unfavorable cervix often requires a long hospitalization before delivery and may result in cesarean section. There has been a considerable interest in the use of nitrous oxide donors for cervical ripening in the last year, especially for its lower complications. This study was conducted to evaluate the efficacy of vaginal isosorbide mononitrate (IMN) for cervical ripening and labor induction.METHODS: A double-blind, randomized, placebo-controlled trial was conducted with 72 nuliparous women with singleton postdated pregnancy and unfavorable cervix. They randomly assigned to receive either 20mg IMN (n=36) or placebo (n=36) tablets vaginally twice each 12h prior to admission for induction of labor. The outcomes were change in Bishop score, mean time from admission to active phase of labor, admission to delivery interval, and presence or absence of fetal and maternal morbidity and the rate of cesarean section that compared in both groups. FINDINGS: IMN was more effective than placebo in inducing a change in Bishop score (3.69±0.89 vs 1.05±0/67 p=0.001), the mean time to active phase of labor was 2.82±0.67h in the IMN group and 3.33±0.55 h in placebo group (p=0.01). IMN shortened the admission to delivery interval as compared with placebo (6.61±3.74 vs 11.48±3.55 h p=0.001). There was no statistical difference in terms of maternal-fetal complications or cesarean section rates in two groups.CONCLUSION: Using of vaginal IMN in an outpatient setting is more efficient than placebo in cervical ripening and shortening of induction-labor interval without increasing maternal-fetal complications, or cesarean section rate.