A rare case of acute myelocytic leukemia that relapsed in association with the formation of a gingival tumor is presented. The patient was a 45-year-old man who had been referred to our department for oral examination before starting chemotherapy. No gingival swelling was found on the initial visit. However, about 4 months after complete remission was achieved, he noticed a gingival tumor arising in the left side of the mandible. The tumor was 47 × 32 mm in size and elastic hard, with a relatively well defined border. Pathologically, the lesion was diagnosed as leukemic cell infiltration, despite the hematologic remission. The patient died of respiratory failure about 3 months after presentation, with no response to chemotherapy.