BACKGROUND Imatinib mesylate has shown promising results in chronic myeloid leukemia (CML) in all phases. This drug is an effective treatment for patients with CML in chronic phase as it induces hematological remission in nearly all patients and cytogenetic responses in many. The bone marrow changes produced by this drug are different from the treatment modalities used earlier in CML. METHODS We studied 80 patients of CML on treatment with Imatinib at doses of 400-800 mg per day. Morphological and cytogenetic evaluation (Ph analysis) of bone marrow aspirates was done at six months of treatment. RESULTS In our study, 95% (76 out of 80) patients showed complete hematological response and 63.3% showed major cytogenetic response at the end of six months of treatment. The most commonly observed changes in the bone marrow aspirates at the end of six months of therapy were in the form of reduction in the cellularity, reduction in the M: E ratio to a mean of 2:1, presence of relative erythroid hyperplasia, normalization of megakaryocytic morphology and variable increase in the bone marrow lymphocytes. None of these changes had significant correlation with the patient's Ph status. CONCLUSIONS We advise study of trephine biopsies to overcome the often-faced problem of hemodiluted aspirates in these cases and evaluation of sequential bone marrows to check the durability of these morphological changes and their correlation with the cytogenetic response with emphasis on cytogenetic changes other than Ph positivity.