Acute Leukemias Patients with an acute leukemia often present with signs and symptoms of bone marrow failure, such as fatigue and pallor, fever, infection and/or bleeding with purpura and petechiae. In acute leukemias, the marrow is typically overpopulated with blast cells. These cells are indistinguishable from stem cells by light microscopy, but the term "blast" implies an acute leukemic clone. The maturing normal marrow cellular elements are decreased or absent. Peripheral leukemic cell counts may range from leukocytosis to leukopenia, but, as anticipated, anemia and thrombocytopenia are common. The acute leukemias are broadly divided into two classes based on the cell of origin: acute lymphocytic leukemia and acute nonlymphocytic leukemia. The previous designation of "acute myeloid leukemia" has been replaced by "acute nonlymphocytic leukemia" to appropriately encompass the full variety of possible abnormal cells (undifferentiated, myeloid, monocytic and megakaryocytic). Acute lymphocytic leukemia most commonly occurs in children less than 18 years of age. Adults usually have acute nonlymphocytic leukemia. Occasionally, patients with acute lymphocytic leukemia have a mediastinal mass or central nervous system involvement at the onset of illness.Blast cells are often seen in the peripheral blood smears of patients with acute leukemia. Auer rods, as shown in Figure 1, are a marker of acute nonlymphocytic leukemia. Because Auer rods do not appear frequently, precise distinction between acute lymphocytic leukemia and acute nonlymphocytic leukemia usually cannot be accomplished based on the peripheral smear alone; histochemistry, immunotyping and chromosome analysis are usually required. All patients with acute leukemia require prompt attention and therapy. White blood cell counts in excess of 100,000 per mm3 (100 X 109 per L) constitute a medical emergency because patients with this degree of leukocytosis are predisposed to brain infarction or hemorrhage.