Standardized definitions of response and outcome are useful to evaluate study data, facilitate treatment guideline adherence and aid clinical decision-making. Although definitions of response in chronic myeloid leukemia (CML) have been standardized, definitions of disease progression have not been standardized, despite widespread adoption of the term to reflect development of advanced disease. This article reviews definitions of response and disease progression used for progression-free survival and event-free survival in key CML studies and implications for interpreting patient response. It is proposed that adherence to definitions of progression from the European LeukmiaNet may aid future research and therapeutic decision-making.