Hydatidiform mole is the most frequently-occurring gestational trophoblastic disease (GTD). Patients with GTD have elevated human chorionic gonadotrophin (HCG) produced by the trophoblast. After evacuation of the mole, weekly serum HCG determinations can be used to assess whether the trophoblast is persisting or regressing based on the standardised normal Nijmegen serum HCG regression curve. The serum HCG pattern is used to establish the diagnosis 'persistent trophoblastic disease'. Treatment with monochemotherapy in the form of methotrexate is the treatment of choice. For about 80% of women with a persistent trophoblastic disease following a molar pregnancy this treatment will lead to a complete remission of the disorder. If resistance to methotrexate occurs then a combined therapy is indicated. A recently developed, and as yet unvalidated, normogram for the serum HCG level during methotrexate treatment might in the near future be able to identify patients with methotrexate resistance at an early stage. Hysterectomy or curettage may be indicated as part of therapy in selected patients. The incidence of choriocarcinoma after a live birth is estimated at 1 in every 40,000 full-term pregnancies. These patients have a high risk for resistance to methotrexate (75%). Treatment of these patients with primary combination therapy at a specialised medical centre is then indicated.