The expression of oestrogen receptor (ER) protein in invasive carcinoma of the breast and its clinical significance has been extensively evaluated. Little information is available regarding ER expression in ductal carcinoma in situ (DCIS). In this study, 46 formalin-fixed, paraffin-embedded tissue specimens of mammographically detected DCIS were evaluated immunohistochemically for the presence of ER using specific monoclonal antibodies against ER (ER-ICA Abbott Lab). The associations between ER expression and histological type, degree of differentiation and patient menopausal status were evaluated. Positive ER staining was present in 72% of cases. Non-comedo types of DCIS were more frequently ER-positive than comedocarcinoma. ER-positive tumours were inversely correlated with the presence of nuclear pleomorphism. The incidence of ER in pre-menopausal and post-menopausal women was similar. In conclusion, ER expression is present in a considerable percentage of DCIS, and ER-positivity is associated with the degree of differentiation and non-comedo carcinoma variants.