Background: Adjuvant endocrine therapies such as tamoxifen, goserelin, and oophorectomy improve survival for premenopausal women diagnosed with early-stage breast cancer. However, these treatments often result in menopausal symptoms, sexual dysfunction, permanent infertility, or the need to delay pregnancy. We aimed to quantify the survival gains that premenopausal patients with early-stage breast cancer require to justify the side-effects and inconvenience of adjuvant endocrine treatments. Methods: Participants consisted of 102 women who had been diagnosed with early-stage (stage I-II) breast cancer 6-60 months previously, who were aged 40 years or younger at diagnosis, and who had been treated for a minimum of 3 months with endocrine therapy (67 with tamoxifen alone, seven with goserelin alone, and 28 with tamoxifen and goserelin or oophorectomy). 76 patients also received chemotherapy, and 75 received radiotherapy. Participants attended a face-to-face patient-preference interview, in which they were presented with four hypothetical clinical scenarios that were used to quantify the gains in survival rate and life expectancy that women judged necessary to make their endocrine therapy worthwhile. They also completed a questionnaire on standard psychological measures. Findings: About half of participants thought that adjuvant endocrine therapy was worthwhile for an absolute gain in survival of 2% from a baseline of either 65% or 85%, and for a gain in life expectancy of 3 months from a baseline of 5 years and of 6 months for a baseline of 15 years. Women who had had more severe endocrine side-effects required larger gains to make endocrine therapy worthwhile (univariate p=0·02, multivariate p=0·04). Interpretation: Modest gains in survival are sufficient to make adjuvant endocrine treatment worthwhile for premenopausal women with early-stage breast cancer. Knowing and incorporating what women think should enhance shared decision-making.