Choriocarcinoma is a malignant germ cell tumor that usually arises from a previous gestation, but may also arise from germ cells anywhere along their known migratory pathway during fetal development. Gestational choriocarcinoma is highly sensitive to chemotherapy. This malignancy is known to undergo spontaneous regression of the primary tumor, which, in the face of metastases, may obscure the primary tumor site. The authors report the case of a patient with choriocarcinoma who was seen with pulmonary metastases and a single large lesion in the kidney 5 years posthysterectomy. The problems in resolving the primary site and the importance of a tissue diagnosis before nephrectomy are discussed.