The administration of chemotherapy following hydatidiform mole evacuation implies a diagnosis of trophoblastic neoplasia. A review of the literature and an analysis of a questionnaire sent to physicians treating trophoblastic disease shows that the criteria used to make this decision vary significantly. The factors that make it important to arrive at a consensus are discussed. Alternative investigations other than hCG that may distinguish neoplasia such as doppler flow ultrasound and magnetic resonance imaging are examined critically. A definition of neoplasia is presented that relates tumor load as measured by hCG to the length of hCG plateau. The changing criteria for distinguishing metastasis are discussed.