Two different receptors which bind angiotensin II specifically have been identified in humans and were designated angiotensin II type-1 receptor (AT1) and angiotensin II type-2 receptor (AT2). They only have 34% sequence homology and act through different signalling pathways. AT1 stimulation has been implicated in hypertrophy and hyperplasia in various tissues. In order to study the involvement of AT1 in tissues from controls (n=10) and patients with hyperplasia (n=33), ductal carcinoma in situ (DCIS) (n=23) and invasive carcinoma of the breast (n=25), we tested biopsies and breast-derived cell lines using immunocytochemistry, in situ hybridisation and cell proliferation techniques. The results show specific overexpression of AT1 receptor on the cytoplasmic membrane of cells of hyperplastic lesions with and without atypia and on DCIS of the breast. Evidence for growth stimulation is provided by in vitro experiments showing growth induction by angiotensin II of T47D cells which express the AT1 but not the AT2 receptor. The expression of AT1 on the cell membrane disappears in invasive breast cancer cells suggesting a regulatory pathway which is no longer needed in invasive carcinoma. The specific AT1 expression upregulation might well be an important step in the pathogenesis of hyperplasia of the breast, which is regarded as a precursor lesion for breast cancer.