Imatinib is a relatively specific inhibitor of the BCR/ABL tyrosine kinase, effective in chronic myeloid leukemia (CML). The aim of our study was to analyse the frequency and type of hematological and non-hematological adverse events in our series of late and early chronic phase patients with CML treated with imatinib and correlate the grade of hematological toxicity with the response obtained. Hematological events were seen in 59 out of 150 (39%) late chronic phase (CP) patients: of these, 24% experienced toxicity Grade 3-4. Of the 100 early CP patients, 26 (26%) had hematological adverse event: 7% experienced toxicity Grade 3-4 (p=0.0001). We found that only in early CP patients, the occurrence of hematological side effects of any grade within 6 months of therapy had a negative influence on cytogenetic response. We compared the incidence of non-hematological adverse events occurring in late and in early CP patients and found that in these latter, some side effects were more frequent, such as weight gain, periorbital edema, muscle cramps, skin rashes, diarrhea, weeping. On the contrary, we found that bone pain and hemorrhagic events were more common in late CP patients. Grade 3-4 adverse events were recorded at rates below 4% and decreased over time: in late CP patients hemorrhages and muscle cramps were the most common side effects of Grade 3-4, whereas in early CP patients, the most frequent events were nausea, weight gain and cutaneous rash. We have observed that hematological and non-hematological side effects during imatinib therapy are different among late and early CP patients and that severe hematological toxicity may influence cytogenetic response only in early CP patients.