Deletions of the 5'ABL region adjacent to the t(9;22)(q34;q11) have recently been reported in 8-32.7% of patients with chronic myeloid leukemia (CML). The deletions were visualized with fluorescence in situ hybridization using, in the majority of the cases, the Vysis LSI BCR/ABL ES (extra signal) probe. In our series, 10 of 99 CML patients (10.1%) were characterized by a 5'ABL deletion. We show that 3'BCR losses are observed in nearly all the cases with 5'ABL deletions. Moreover, the different genetic events (Philadelphia chromosome formation; 5'ABL and 3'BCR deletions) occur simultaneously in a one-step process without any evidence for genetic instability in the target bone marrow cells.