Aims: This study aimed to identify the optimal human chorionic gonadotropin (hCG) ratio in predicting etoposide, methotrexate, dactinomycin, cyclophosphamide, and vincristine resistance in women diagnosed with high-risk gestational trophoblastic neoplasia (GTN) and to compare the chemoresistant disease detection rate by using the optimal hCG ratio and traditional criteria.Methods: Seventy-six women with primary high-risk GTN treated with etoposide, methotrexate, dactinomycin, cyclophosphamide, and vincristine in a tertiary-care center were included. The hCG ratio was determined by its serum pretreatment level divided by that before each cycle of chemotherapy. The traditional criteria for chemoresistance included plateau or rising of hCG or presence of new metastasis.The optimal hCG ratio was determined using receiver operating characteristics (ROC) curve analysis.Results: Among the specificities of 90%, 92.5%, and 95%, the 90% specificity yielded the best ROC curve. At 90% specificity, the best area under curve value was at the fourth cycle with 75% sensitivity. The hCG ratio at the fourth cycle was 31.92. Using the ratio at the fourth cycle, chemoresistant disease was detected in six out of eight patients, compared to one in the traditional criteria. When combining the two diagnostic tools, the cumulative detection rate in the fourth cycle was 10/12 (83.3%) of total drug resistance. Among patients who developed drug resistance at the fourth cycle or thereafter, the use of the ratio at the fourth cycle could diagnose chemoresistance approximately two cycles earlier than that with the traditional criteria.Conclusions: A hCG ratio of <31.9 at the fourth cycle should be considered a highrisk for etoposide, methotrexate, dactinomycin, cyclophosphamide, and vincristine resistance and may need second-line chemotherapy. The ratio increases the detection rate of resistance to these drugs more than the traditional criteria.