Objective: The aim of the study was to determine whether isosorbide mononitrate (ISMN) 60 mg administered vaginally is effective for preinduction cervical ripening on an outpatient basis. Material and methods: The study was carried out at Karnataka Institute of Medical Sciences, Hubli, Karnataka from November 2007 to October 2008. Hundred women with singleton pregnancies with Bishop score =6 were randomized to receive either ISMN slow-release (ISMN-SR) 60 mg or vitamin C vaginally on an outpatient basis. Bishop score, proportions establishing spontaneous labor were assessed after 48 hours. Requirement of additional cervical ripening agent, need for oxytocin, admission-delivery interval, neonatal outcome (Apgar score 5 minutes, neonatal intensive care unit [NICU] admission and meconium-stained liquor) and side effects were compared. Results: In ISMN-SR group, there was a marked increase in the proportion of women establishing spontaneous labor (36% vs. 12%) and being favorable for induction of labor (40% vs. 9.09%). There was a significantly higher Bishop score (6.76 ± 2.65 vs. 4.6 ± 2.17) and decrease in proportion of subjects requiring further ripening (38% vs. 80%). Admissiondelivery interval was shorter in ISMN group (p < 0.001). There were no significant differences in mode of delivery and fetal distress. Headache was seen in 22% of women in the ISMN group (p < 0.01). Conclusion: ISMN administered vaginally is effective for preinduction cervical ripening.