Chronic myeloctyic leukemia (CML) is a stem cell disorder characterized by the cytogenetic abnormality of t(9;22)(q34;q11.2), which progresses from a chronic phase to an accelerated phase (AP), and/or a blast phase (BP) of myelocytic or lymphoid phenotype. This progression is frequently preceded or accompanied by recurring secondary chromosomal abnormalities (SCA) that are believed to play a role in the transformation and may also serve as valuable prognostic indicators. Failure to note such abnormalities may lead to an inappropriate clinical evaluation. We observed CML patients with AP or BP who did not show SCA by routine cytogenetic analysis. To determine the presence or absence of specific SCA in those cases, we applied fluorescence in situ hybridization (FISH) to four CML cases with pseudodiploid cytogenetics [t(9;22)(q34;11.2) as the sole abnormality] by conventional karyotyping. Bone marrow biopsies from two AP and two BP of CML patients with pseudodiploid karyotypes by conventional cytogenetics were examined by FISH for trisomy 8 and i(17q). These SCA are major secondary chromosomal changes seen in BP of CML patients. Results were considered positive if more than 2.4% of cells had +8 and >6.25% for i(17q) by FISH. Four out of four patients were positive for +8. These results indicate that FISH techniques are valuable in the determination of SCA in CML, which were t(9;22)(q34;q11.2) positive as the sole cytogenetic abnormality with standard G-banding karyotyping and can be helpful for the early diagnosis of CML progression.