A 56-year-old man was admitted to the hospital in April, 1986, with the chief complaint of fatigue. The diagnosis of acute promyelocytic leukemia (APL) was made based on the proliferation of atypical promyelocytes in the bone marrow. No gingival swelling was found on admission. A complete remission was achieved by the BHAC-DMP therapy and maintained by the consolidation therapy and the intensification therapy. In July, 1987, he noticed a solitary gingival tumor around the left lower second molar. The biopsy showed the massive infiltration of leukemic cells, despite the hematological remission. Combination chemotherapy was not effective but the tumor disappeared by irradiation. A hematological relapse occurred in November, 1987, but the second complete remission was achieved by the AB-triple V therapy. He died because of the second hematological relapse in July, 1988. Along with the wide use of intensive chemotherapy for acute leukemia, tumor forming leukemia during the hematological remission, as seen in this case, would be increasing. Thus, we should not overlook any newly formed tumor in the treatment of leukemia.