We examined the influence of hormone replacement therapy (HRT) on breast tumour biology by comparing the prognostic characteristics of breast cancers and survival in 121 women prescribed replacement hormones before diagnosis with those in 1468 women without such treatment. The women receiving HRT had a lowered relative risk of being diagnosed with tumours of more than 20 mm in diameter, OR = 0.7 (CI 0.5-1.0) and axillary lymph node dissemination, OR = 0.7 (CI 0.4-1.1). These risk reductions were most pronounced and statistically significant in the women who had been prescribed a combined estradiol-progestin regimen. The patients in this compound group also had a diminished relative risk of having poorly differentiated tumours. Further, there was an indication that the women prescribed HRT, and especially those with conjugated estrogens/estradiols alone, had a decreased relative risk of developing aneuploid tumours. There was no clear pattern for women receiving the biologically weak oestriol, although risk estimates were generally higher for unfavourable tumours in comparison with those receiving the higher potency compounds. Adjustments for indications of earlier detection (i.e. lead time bias) did not influence the pattern or magnitude of the risk estimates. No association between any type of HRT and survival after breast cancer diagnosis was noted, but analyses were based only on 19 breast cancer deaths among exposed patients. We conclude that breast cancers occurring after treatment with HRT, especially the combined estrogen-progestin regimen, seem to have more favourable tumour features than tumours in non-treated women. Our findings may reflect a less aggressive biological behaviour of breast cancers in women receiving HRT, or in part be explained by the earlier detection of the tumours in these women.