Objective To evaluate the predictive factors associated with the success of medical abortion by misoprostol monotherapy within 24 hours in the first trimester of pregnancy. Methods The records of 228 women with miscarriage up to 11 weeks of gestational age who underwent medical abortion by intravaginal misoprostol monotherapy were reviewed. Success of abortion was defined as complete expulsion of the conceptus without the need for surgical intervention. Outcomes of interest were success of abortion within 24 hours following administration of misoprostol. Results Among 222 women who continued the process of medical abortion for 24 hours, 209 (94.1%) had a successfully completed abortion. Multivariate logistic regression showed that serum β-human chorionic gonadotropin (β-hCG) above 40,000 mIU/mL is significantly associated with failed medical abortion within 24 hours (odds ratio [OR], 7.13; 95% confidence interval [CI], 1.60-37.32; P =0.011). The area under the receiver operating characteristic curve of β-hCG level associated with successful abortion within 24 hours was 0.705 (95% CI, 0.63-0.77; P =0.007). Previous vaginal delivery seems to be significantly associated with successful abortion within 24 hours on univariate analysis (P =0.037), but the association was lost in multivariate analysis. Conclusion Misoprostol monotherapy has a high success rate for first trimester abortion. Women with serum β-hCG less than 40,000 mIU/mL are likely to achieve a successful abortion within 24 hours after intravaginal administration of misoprostol.