Chromosomal abnormalities in Ph-negative metaphases from patients with chronic myeloid leukemia (CML) treated with imatinib have been described in some cases. Trisomy 8 is the most frequent, but monosomy 7 has also been described. However, the association of these chromosomal alterations with myelodysplasia has been scarcely reported. We report the appearance of monosomy 7 in Ph-negative cells, associated with severe dysplasia, in two patients with CML treated with imatinib, with a different outcome: one with a transient evolution and the other evolving to acute myeloid leukemia.