Molecular methods are emerging as important tools for the diagnosis and stratification of patients with leukemia. Together with rearrangements in immunoglobulin and T-cell receptor genes, balanced translocations are the most important genetic lesions amenable to molecular diagnosis. Moreover, many publications have identified significant differences in the prognosis of acute leukemia patients with such balanced translocations. Because not all balanced translocations can be diagnosed by cytogenetic techniques, reverse-transcriptase polymerase chain reaction (RT-PCR)-based methods are increasingly employed. This method has the added advantage that it can also be used to monitor for minimal residual disease (MRD). A disadvantage is that the multitude of balanced translocations in leukemia would make efforts to detect all lesions at diagnosis by such standard techniques extremely labor-intensive. Furthermore, difficulties in optimizing semiquantitative PCR assays have limited the utility of these methods for MRD. Recent advances in the design of multiplex PCR, which detects a number of genetic aberrations simultaneously, may improve the diagnostic process. Accurate quantitation of the fusion transcript for balanced translocations has become possible by use of fluorogenic probes and real-time PCR. Together, such methodologies may constitute a novel platform for the integration of molecular methods in clinical decision-making. (C) 2000 by W.B. Saunders Company.