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Four new cases of primary choriocarcinoma arising in the placenta during a seemingly normal gestation were studied at the Trophoblastic Disease Center of Northwestern University. In each case the patient presented with disseminated metastases while carrying an intrauterine gestation with a normally developing fetus. All four placental primaries were small; three of the tumors were microscopic and found only after extensive sectioning. Histologically, these tumors all appeared to arise from the cytotrophoblastic cells covering the stromal portion of villi, and in some areas the involved villi retained a portion of normal investing trophoblast. This study shows that gestational choriocarcinoma unassociated with hydatidiform mole can have an early stage in which chorionic villi are present. The consistently small size of the lesions studied suggests that primary placental choriocarcinoma may frequently be overlooked or missed, and that choriocarcinoma possibly has its origin in the placenta more often than in retained or persistent trophoblast following pregnancy.