A survey has been made of all invasive moles and gestational choriocarcinomata registered during a 30-year period in Denmark, a nation with a population of about 5 million (1971). All the diagnoses have been verified histologically by reappraisal. Based on clinical data the 2 groups of patients are described with reference to the following criteria: incidence and age; gravidity, parity, and latency; dissemination and course of disease; and treatment. Some case histories are reported which contrast with the view generally prevailing before the era of chemotherapy, viz. that choriocarcinoma was a highly malignant and rapidly progressive disease. All the 19 patients who had an invasive mole, including 5 with metastases, became free from symptoms, and transition from this lesion to choriocarcinoma did not occur in any of these cases. Among the 50 choriocarcinoma patients the over-all mortality was 64%; 18 patients (36%) —10 of them with metastases—recovered. A decreasing mortality was observed during the period under investigation, but much additional work has to be done before the prognosis of the disease approaches that of invasive mole. In conclusion, the results of this study illustrate the many difficulties encountered in the diagnosis, treatment and follow-up of these rare trophoblastic tumours. The need for collaboration in prospective cases and for centralized treatment is strongly indicated from scientific as well as therapeutic points of view.