The molecular analysis of acute lymphoblastic leukemia (ALL) has provided exciting insights into the pathogenesis of this disease. This disease is heterogenous and can be subtyped based on chromosomal, immunophenotypic, and structural criteria. The varying prognostic implications of different ALL subtypes markedly influence the treatment decisions in adults. Many patients with T-cell ALL can be cured with chemotherapy alone. In contrast, patients with early B-lineage ALL with certain chromosomal abnormalities, especially the Philadelphia chromosome, do not have durable responses to chemotherapy and should receive a bone marrow transplantation if an HLA-matched donor is available. Recent reports have shown improved results for adults with B-cell ALL (Burkitt's) after intensive alternating cycles of chemotherapy containing high doses of methotrexate and cyclophosphamide. Future clinical and laboratory investigation should lead to the development of novel and possibly more effective treatments specifically tailored for different subsets of ALL.