This study will evaluate the efficacy, safety and tolerability of long-term use of peginterferon alfa-2a in participants with CML who have previously participated in peginterferon alfa-2a study ML16544 (NCT number not available), NO16006 (NCT number not available) or ML17228 (NCT number not available) and treating physician has decided to continue treatment with peginterferon alfa-2a within the frame of another clinical study.