We report 5 cases of chronic myelogenous leukemia (CML) and 1 case of acute myeloid leukemia (AML) with the dual presence of t(9;22) and inv(16). The 6 patients were 5 men and 1 woman with a median age of 42.5 years. All cases were BCR-ABL+ with p210 products detected in all CML cases and a p190 product detected in the AML case. An increase in bone marrow eosinophils was detected in 3 of 5 cases, and abnormal eosinophils were identified in these 3 cases. The CBFbeta-MYH11 fusion gene was confirmed in all 3 CML cases and the 1 AML case tested, and this correlated with the presence of abnormal eosinophils with coarse basophilic granules. Of 5 patients with CML, 4 had a rapid transformation to myeloid accelerated phase of blast crisis. The coexistence of t(9;22) and inv(16) in CML seems to correlate with more rapid transformation.