Objective. To study the potential of long interspersed element-1 (LINE-1) methylation change in the prediction of postmolar gestational trophoblastic neoplasia (GTN). Methods. The LINE-1 methylation pattern from first trimester placenta, hydatidiform mole, and malignant trophoblast specimens were compared. Then, hydatidiform mole patients from 11999 to 2010 were classified into the following 2 groups: a remission group and a group that developed postmolar GTN. Specimens were prepared for a methylation study. The methylation levels and percentages of LINE-1 loci were evaluated for their sensitivity, specificity, and accuracy for the prediction of postmolar GTN. Results. First, 12 placentas, 38 moles, and 19 malignant trophoblast specimens were compared. The hydatidiform mole group had the highest LINE-1 methylation level (p = 0.003) and the uCuC of LINE-1 increased in the malignant trophoblast group (p ≤ 0.001). One hundred forty-five hydatidiform mole patients were classified as 103 remission and 42 postmolar GTN patients. The %mCuC and %uCmC of LINE-1 showed the lowest p value for distinguishing between the two groups (p < 0.001). The combination of the pretreatment β-hCG level (≥100,000 mIU/mL) with the %mCuC and %uCmC, sensitivity, specificity, PPV, NPV, and accuracy modified the levels to 60.0%, 92.2%, 77.4%, 83.8%, and 82.3%, respectively. Conclusions. A reduction in the partial methylation of LINE-1 occurs early before the clinical appearance of malignant transformation. The %mCuC and %uCmC of LINE-1s may be promising markers for monitoring hydatidiform moles before progression to GTN.