As oestrogen is associated with most of the epidemiological risk factors for breast cancer, the number and distribution of oestrogen receptor positive (ER+) cells could have a bearing on the development of the disease. ER+ cells were thus studied in the normal breast and in the spectrum of in situ proliferations which range from non-atypical hyperplasia to in situ carcinoma and are associated with different levels of risk for developing breast cancer. In the normal pre-menopausal breast, ER+ cells comprised the minority and were distributed singly, being surrounded by oestrogen receptor negative (ER ) cells. ER+ cells showed a statistically significant increase with age, reaching a plateau after the menopause, and the increase was associated with a tendency for positive cells to become contiguous in patches of variable size. A small proportion of lobules showing involutional change comprised over 90 per cent ER+ cells. The significance of this feature is not clear but no evidence was found that it was pre-cancerous. The percentage of ER+ cells was slightly increased in hyperplasia of usual type (non-atypical hyperplasia, HUT) and the relationship to age was maintained. The staining pattern was variable; in some lesions ER+ cells were surrounded by ER cells whereas in others there were contiguous groups of positive cells sometimes accounting for more than 90 per cent of cells in the lesion. In contrast, all cases of atypical ductal hyperplasia (ADH), lobular in situ neoplasia (LIN) and ductal carcinoma in situ (DCIS) exhibited positivity of contiguous cells accounting for the majority in the lesions. Furthermore, the relationship between ER+ cell numbers and age was lost in these lesions, indicating autonomy of ER expression or of proliferation of cells expressing the receptor. It is hypothesized that this dysregulation of receptor expression or of ER+ cell numbers at the ADH stage may be the precursor of abnormal expression of cyclins and other cell cycle control proteins which have been shown first to appear in DCIS.