Leukemia is a hematological disorder arises from a hematopoietic stem cell characterized by a disordered differentiation and proliferation of neoplastic cells. Rapidly forming generalized gingival hyperplasia is usually the first sign of this disease (especially in acute forms). This case report describes a 54-year-old female who presented rapid gingival enlargement in only three weeks time, heralding the presence of acute monocytic leukemia (AML-FAB M5). The patient was immediately referred for hematologic management, but died five weeks later. Dentists and generalists should always be on guard to observe any oral manifestations that may lead to the early diagnostic of systemic diseases.