Sixty % of CML patients treated by Imatinib mesylate achieved a major cytogenetic responses (CCR) at 18 months. So, 40% of the patients must receive additional treatment. In vitro, it has been shown that IM and Interféron-alpha have synergic anti-proliferative effect on chromosome Ph+ cell lines. By using Peg-Interféron and IM combination, we hope to increase the cytogenetic response of patients.

Inclusion Criteria:

• Chronic phase Philadelphia positive CML, confirmed by karyotyping (or FISH) analysis, in complete hematologic response with IM.
• Lack of major cytogenetic response after at least one year of STI 571 as single therapy.
• Male and female * 18 years old.
• Informed consent signed up.
• Performance status grade 0 - 2 (ECOG).
• SGOT and SGPT <3N
• Serum bilirubin < 1.5 N
• Serum creatinine < 1.5 N
• No HSC graft planned
• B-HCG negative for female with potential childbearing

Exclusion Criteria:

• Absence of complete hematologic response
• Extramedullar involvement
• Previous extra-hematologic intolerance of Interféron at a dose superior or equal to 25 MUI/week
• Depressive syndrome not controlled
• Not controlled dysthyroidy
• Auto-immune pathology not controlled
• Women with childbearing potential who are unwilling or unable to use an adequate method to avoid pregnancy for the entire period of the study
• Significant cardiac disease (grade 3 or more)
• Known seropositivity for HIV
• Active viral hepatitis
• Other malignant disease
• Other experimental medication