BACKGROUND AND OBJECTIVE: The emergence of non-random chromosomal abnormalities is a well-recognized occurrence in chronic myeloid leukemia (CML) and detection of these abnormalities is important in prognostic stratification. The frequency and types of additional chromosomal abnormalities in CML patients has not been determined in our region. PATIENTS AND METHODS: We conducted a descriptive, prospective study of additional chromosomal abnormalities in patients with an established diagnosis of Philadelphia-positive CML from May 2001 to June 2007. Cytogenetic studies were repeated every three months with the conventional G-banding technique and described according to the international system for Human Cytogenetic Nomenclature. All patients received imatinib mesylate. RESULTS: In 219 patients with Philadelphia-positive CML, 34 (15.5%) (median age, 38 years) developed 51 additional chromosomal abnormalities. Five cases had variant translocations prior to starting imatinib; the remaining 29 cases acquired chromosomal abnormalities after starting imatinib, including 8 cases that received prior interferon-alfa. Twenty-one patients were in chronic phase, 10 in accelerated phase and 3 were in blast crisis. Trisomy 8 was the most frequent abnormality followed by random chromosomal abnormalities and variants of the Philadelphia chromosome. CONCLUSIONS: The overall frequency of additional chromosomal abnormalities was similar to that in previous reports. Early identification of these abnormalities may help in adapting to a more appropriate therapeutic approach.