OBJECTIVES: The association between pre-diagnostic hormone replacement therapy (HRT) and breast cancer specific mortality as well as potential influences from other lifestyle factors on the association was investigated. STUDY DESIGN: Female participants from the prospective cohort "Diet, Cancer, and Health" diagnosed with breast cancer (BC) were identified and their pre-diagnostic HRT use evaluated for association with tumour biology and breast cancer outcome in multivariate analysis. MAIN OUTCOME MEASURE: Breast cancer specific mortality. RESULTS: Of the 1212 patients originally considered 1064 were included. Of these, 105 women died from breast cancer during a median follow-up of 6.3 years (range 0.2-14.3 years). In multivariate analyses women who used HRT at enrolment into the cohort study had 47% lower risk of dying from breast cancer as compared to women who had previously or never used HRT (adjusted HR: 0.53; 95% CI, 0.37-0.85). Pre-diagnostic HRT use was associated with smaller tumour size at the time of diagnosis and a higher frequency of receptor positive breast cancer. Paradoxically, a high pre-diagnostic intake of vitamin D supplements was associated with HRT use but also with a higher BC specific mortality (HR: 1.47; 95% CI, 1.07-2.00) CONCLUSIONS: HRT use at enrolment was associated with breast tumours of smaller size at the time of diagnosis and positive receptor status, and with a lower BC mortality. The found association between vitamin D from supplements and higher BC mortality warrants further exploration.