This is a phase III multicenter, open-label study designed to investigate the efficacy (hematological response, cytogenetic response and molecular response) and feasibility (tolerance, compliance and safety) of the tyrosine kinase inhibitor imatinib mesylate (formerly STI 571, GLIVEC, Novartis Pharma) at conventional dose (400 mg/daily) if compared with high dose (800 mg/daily) (serial number protocol ICSG/CML/022) in patients with Ph+ chronic myeloid leukemia (CML) in chronic phase (CP) previously untreated, at high Sokal risk.

Inclusion Criteria:

1. Age >/=18 years
2. First chronic phase, less than 6 months of duration
3. High Sokal's risk
4. Ph positive
5. No previous treatment or hydroxiurea only.
6. Performance status (ECOG/WHO) < 2
7. Written informed consent

Exclusion Criteria:

1. Age <18
2. Low or intermediate Sokal risk score.
3. More than 6 months from diagnosis.
4. Second chronic, accelerated or blastic phase
5. Scheduled allogeneic stem cell transplantation within 1 year from diagnosis.
6. Performance status (ECOG/WHO) > 2
7. Inability to provide written informed consent
8. Pregnancy
9. Formal refusal of any recommendation of a safe contraception
10. Alcohol or drug addiction
11. Altered hepatic or renal function as defined by AST/ALT or bilirubine > 3 times upper normal limits (UNL) and by creatinine > 20mg/L
12. Any other disease or condition that by the advise of the responsible physician would make the treatment dangerous for the patient or would make the patient ineligible for the study, including physical, psychiatric, social and behavioural problems.