Current evidence strongly implicates the chromosome translocation t(9;22)(q34;q11.2) as the cause of chronic granulocytic leukemia. Therefore, identification of this genetic abnormality through either cytogenetic or molecular methods has become a requirement for diagnosis. Intense investigation of the mechanism by which t(9;22) transforms normal hematopoietic progenitors into malignant cells is ongoing. Recent advances in molecular diagnostic methods have allowed refined qualitative and quantitative methods of detecting t(9;22), which are useful for monitoring response status and detecting minimal residual disease. The current understanding of the pathogenesis of chronic granulocytic leukemia and the application of new diagnostic methods are discussed.