Cytogenetic and molecular studies have assumed an increasing role in the evaluation and management of patients with leukemia. Many cytogenetic findings have become well established as important independent indicators of prognosis in the acute leukemias. There has been a recent explosion of knowledge about the genes involved in leukemogenesis and the manner in which their structure or expression is altered by chromosomal translocations or point mutations. Application of newer techniques such as the polymerase chain reaction and in situ hybridization has begun to allow quantitation of residual leukemia cells after therapy. Cytogenetic and molecular findings should allow us to use more individualized therapy in treating leukemia, as well as therapy targeting leukemia-specific abnormal gene products.