Chronic myeloid leukemia is an enigma in origin and a source of frustration in treatment because until recently no real progress had been made in altering its natural history. Nevertheless, recent advances in cell biology and molecular genetics, and a plethora of morphologic, biochemical, and cytogenetic data of potential clinical relevance have yielded new data regarding this disease. The use of marrow transplantation and recombinant interferon have been of significant clinical importance, although their roles in the overall treatment of the disease remain to be determined.