Recent contributions from molecular biology, biotechnology and recombinant DNA applications have led to important clinical and therapeutic advances in chronic myelogenous leukemia (CML). Developments in the methodology of genetic investigation have clarified the molecular alterations brought about by the appearance of the Philadelphia chromosome. It is possible that the hybrid bcr/abl gene plays an important role in the pathogenesis of CML by subverting the mechanism of homeostasis through the uncoordinated activation of cell growth stimulating and regulating factors. Further improvements have been brought about by the polymerase chain reaction (PCR) which permits an indirect identification of the fusion gene and the study of minimal residual disease during remission after chemotherapy and bone marrow transplantation (BMT). Clinical trials have shown that alpha-interferon, alone or in association with chemotherapy, induces long term clinical and cytogenetic remission in those CML patients in whom BMT from either related or unrelated donors cannot be performed. Allogeneic BMT seems to be the treatment of choice in younger people. However, since a minority of subjects have HLA identical siblings, the possibility of using unrelated donors to provide long term disease free survival has been explored even if the availability of a compatible donor is the primary limiting factor. The development of in vivo and in vitro purging procedures has aroused new interest in autologous bone marrow transplantation. This procedure benefits particularly from biotherapeutic agents which selectively act on the marrow by suppressing bcr/abl positive cells.