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.