Most hematologic malignancies occurring in a patient with a malignant solid tumor are secondary leukemia developing as complications after chemotherapy or radiotherapy. On the other hand, it is very rare for a solid tumor and leukemia to occur simultaneously in a patient without antecedent chemotherapy or radiotherapy. It may be possible that leukemia occurs concurrently with a germ cell tumor because hematopoietic cells and germ cells originate from the yolk sac. In a 3-month-old female diagnosed with acute myelogenous leukemia (AML), a retroperitoneal mature teratoma was found incidentally during the induction chemotherapy. We describe this rare case of concurrent AML and retroperitoneal teratoma in an infant.