Imatinib 400mg daily is the current NICE-approved standard treatment for newly diagnosed Chronic Myeloid Leukaemia (CML). 5 yr follow up of CML patients treated in this way indicates an 89% probability of progression-free survival. Imatinib is not tolerated or effective in some patients however, and a proportion of patients become resistant to the drug. SPIRIT 2 study aims to establish whether a new drug, dasatinib, is superior to imatinib in terms of event free survival and therefore will be an effective first-line therapy for newly-diagnosed CML patients. This study will also provide crucial long-term survival, quality of life and health economic data to assist health care providers and managers to determine the most cost-effective drug therapy for CML.