Acute leukemia is a hematopoietic progenitor cell malignancy. These cells proliferate in an uncontrolled fashion and ultimately replace normal bone marrow elements. Most cases arise with no clear cause. However, radiation and some toxins (benzene) are clearly leukemogenic. In addition, a number of chemotherapeutic agents (especially procarbazine, melphalan, other alkylating agents, and etoposide) may cause leukemia. The leukemias seen after toxin or chemotherapy exposure often develop from a myelodysplastic prodrome and are associated with abnormalities in chromosomes 5 and 7. Although a number of other cytogenetic abnormalities are seen in certain types of acute leukemia, their exact role in pathogenesis remains unclear.

Most of the clinical findings in acute leukemia are due to bone marrow failure, which results from replacement of normal bone marrow elements by the malignant cell. Less common manifestations include direct organ infiltration (skin, gastrointestinal tract, meninges). Acute leukemia is one of the outstanding examples of a once invariably fatal disease that is now treatable and potentially curable with combination chemotherapy.

Acute lymphoblastic leukemia (ALL) comprises 80% of the acute leukemias of childhood. The peak incidence is between 3 and 7 years of age. However, ALL is also seen in adults and comprises approximately 20% of adult acute leukemias. Acute myelogenous leukemia (AML; acute nonlymphocytic leukemia [ANLL]) is chiefly an adult disease with a median age at presentation of 50 years and an increasing incidence with advanced age. However, it is also seen in young adults and children.