AIM: To study the histopathological features of breast carcinoma developing in postmenopausal patients on hormone replacement therapy (HRT). METHODS: The sample comprised 60 patients with invasive breast carcinoma including 31 who had received HRT at or shortly before presentation, and 29 who had not. Details concerning their tumour size, histological type and grade, lymph node status, and oestrogen and progesterone receptor status were compared. Immunoperoxidase staining for Bcl-2, p53, and E-cadherin was carried out on paraffin sections of all 60 patients. The results were then statistically analysed. RESULTS: Tumours detected in HRT patients were significantly smaller (mean 17 mm v 25 mm; p = 0.0156) and of a lower histological grade (p = 0.0414) than those detected in non-HRT patients. The incidence of invasive lobular carcinoma was slightly higher in HRT patients (19% v 14%). Immunohistologically, 87% of HRT tumours were Bcl-2 positive (compared with 79% in the control group), 29% were p53 positive (45% in the control), and 48% were E-cadherin positive (72% in the control group). Although the differences were not statistically significant there was a trend towards higher incidence of p53 negative and E-cadherin negative tumours in HRT patients. CONCLUSIONS: Breast carcinomas detected in patients on HRT have a significantly higher incidence of two favourable prognostic features (small size and a low histological grade). They also show a trend, statistically not significant, of being p53 negative and E-cadherin negative; this may be related to the slightly higher incidence of invasive lobular tumours in these patients.