BACKGROUND: Philadelphia(Ph) chromosome is found in about 95 percent of chronic myelogenous leukemia(CML) patients. Ph chromosome results from a reciprocal translocation between the long arms of chromosomes 9 and 22, and the fusion gene, BCR-ABL contribute to oncogenesis. Three to five years after first diagnosis, CML progresses to the blast crisis, and is accompanied by secondary cytogenetic changes in about 85% of cases. In this study, we investigated the incidence of ABL deletion of derivative 9 chromosome in CML and evaluated the association between this deletion and progression to the blast crisis by interphase fluorescence in situ hybridization(FISH). METHOD: The subjects included in this study were a consecutive series of 58 patients who were diagnosed as CML at Seoul National University Hospital between January 1997 and April 2000. On 90 archival bone marrow aspirate samples from these 58 CML patients, interphase FISH was performed with a commercially available probe. RESULTS: The ABL deletion of derivative 9 chromosome was detected in 17(29.3%) of 58 patients with CML. Eighteen of 58 patients progressed to blast crisis in this period. ABL deletion was found in 7 of 18 patients with blast crisis, and not in 11 remainders. The mean duration from the diagnosis to blast crisis was 37.1 months in 7 patients with the ABL deletion, while the mean duration was 74.2 months in 11 patients without the ABL deletion. The mean duration from the diagnosis to blast crisis in patients with ABL deletion was significantly shorter than in patients without ABL deletion(P=0.043). CONCLUSIONS: We found that 29.3% of patients with CML had the ABL deletion on derivative 9 chromosome. In these patients, the time taken for evolution to blast crisis was significantly shorter than that of the patients without ABL deletion.