Mature outcomes from adjuvant endocrine therapy trials in estrogen receptor-positive breast cancer have enabled comparisons with neoadjuvant clinical trials that have parallel randomizations of treatment in terms of the response of disseminated disease versus the local response within the breast. Imprecise end points, such as 'clinical response', have produced inconsistent results regarding the relationship between neoadjuvant and adjuvant endocrine therapy outcomes. However, the proliferation marker Ki-67, measured during neoadjuvant treatment, has predicted accurately and consistently the results of much larger studies in the adjuvant setting. In this Review, we summarize these trials and discuss the implications for the design of future adjuvant endocrine therapy trials. We conclude that there is sufficient evidence supporting the view that the degree of Ki-67 suppression is a reliable short-term surrogate for the adjuvant potential of endocrine drugs, at least in postmenopausal women. We propose that adjuvant endocrine therapy trials should only be conducted once adequately-powered neoadjuvant studies have indicated superior Ki-67 suppression in patients receiving experimental endocrine treatment versus the standard treatment.