Acute promyelocytic leukemia (APL) is a hematological emergency that requires urgent intervention because of the high incidence of early hemorrhagic death. When patients with APL experience a synchronous solid organ tumor, the tumor’s treatment must also be done properly. Differentiation-inducing therapy using arsenic trioxide (ATO) has less hematological toxicity compared to cytotoxic chemotherapy and might be preferable for untreated APL patients with a synchronous solid organ tumor. Here we describe the first successful case of untreated APL and synchronous endometrial cancer (in an adult Japanese woman) treated with ATO consolidation therapy and the subsequent surgery and chemotherapy for endometrial cancer