PURPOSE: A generally accepted definition for resistance to first-line single-agent chemotherapy for persistent trophoblastic disease (PTD) is lacking. In the present study, a normogram for serum human chorionic gonadotropin (hCG) from patients with normalization of serum hCG after first-line single-agent chemotherapy for PTD was constructed to identify patients resistant to this chemotherapy. PATIENTS AND METHODS: Between 1987 and 2004, data from 2,132 patients were registered at the Dutch Central Registry for Hydatidiform Moles. A normal serum hCG regression corridor was constructed for 79 patients with low-risk PTD who were cured by single-agent methotrexate (MTX) chemotherapy (control group). Another group of 29 patients with low-risk PTD needed additional alternative therapies (dactinomycin and multiagent chemotherapy) for failure of serum hCG to normalize with single-agent chemotherapy (study group). RESULTS: Serum hCG measurement preceding the fourth and sixth single-agent chemotherapy course proved to have excellent diagnostic accuracy for identifying resistance to single-agent chemotherapy, with an area under the curve (AUC) for receiver operating characteristic curve analysis of 0.949 and 0.975, respectively. At 97.5% specificity, serum hCG measurements after 7 weeks showed 50% sensitivity. CONCLUSION: In the largest study to date, we describe the regression of serum hCG levels in patients with low-risk PTD successfully treated with MTX. At high specificity, hCG levels in the first few courses of MTX can identify half the number of patients who are extremely likely to need alternative chemotherapy to cure their disease and for whom further treatment with single-agent chemotherapy will be ineffective.