UNLABELLED: One of the primary adverse effects of long-term use of hormone replacement therapy (HRT) is a modest increase in the risk of breast cancer. Breast tumours that develop in women using HRT have been shown to have prognostically favourable histological features but it is unclear if this is the case for both short- and long-term use. METHODS: We evaluated the association between HRT use with tumour size and histologic grade in a cohort of women aged over 55 years (n = 2200) diagnosed with invasive breast cancer at subsequent screen in BreastScreen Victoria (BSV), Australia between 1993 and 2000. BSV biennially screens women aged over 40 years with the target age group 50-69 years. Multiple linear regression was used to examine predictors of log-transformed tumour size and multinomial logistic regression was used to evaluate associations of HRT with tumour grade. RESULTS: Short-term users of HRT (< or = 5 years), were approximately 50% less likely to develop poorly-differentiated breast tumours OR 0.48 95% CI (0.28-0.82) or node-positive tumours OR 0.57 95% CI (0.35-0.94) than non-users. Long-term users of HRT (> 5 years) were also less likely to develop poorly-differentiated tumours OR 0.36 95% CI (0.24-0.56) but were not more likely to be node-positive than women not on HRT. Duration of HRT use was not significantly associated with tumour size. CONCLUSION: HRT use, regardless of duration, was associated with breast tumours that were better differentiated and not significantly larger than women not on HRT, although only short-term use was associated with fewer node-positive tumours.