The application of molecular biological techniques in malignant hematologic diseases, particularly leukemias, has led to a rapid increase in knowledge and a deeper insight into the pathobiology of these diseases. Leukemia is a very heterogeneous disease on the molecular level. Clonal chromosomal abnormalities can be found in the vast majority of cases. A broader understanding of the underlying molecular changes has enabled the development of risk-adapted therapy regimens with improved outcome. One example is the application of molecular genetic techniques for detecting small numbers of leukemia cells after therapy ("minimal residual disease"). This allows the early preclinical recognition and prevention of relapse. This study gives a short overview of the current status of molecular genetics in human leukemias and its implications for the therapy and prognosis of these diseases. The following topics are covered: generation of oncogenic fusion genes or dysregulation of protooncogenes by chromosomal translocations, detection of minimal residual disease based on clonally rearranged immunoglobulin or T-cell receptor genes or on leukemia-specific fusion genes, monitoring bone marrow chimerism after allogeneic transplantation by molecular genetic techniques, and the role of oncogenic herpesviruses and retroviruses in human hematologic malignancies.