Although estrogen receptor (ER) content and its clinical significance have been extensively evaluated in invasive breast cancer, ER expression in carcinoma in situ (CIS) of the breast and its correlates are less well understood. Thus, using an indirect immunoperoxidase technique and paraffin-embedded tissue, the authors studied ER expression in 100 breast tumors containing CIS with or without invasive carcinoma. The percentages of positive and of strongly positive nuclei were compared among histologic categories of CIS and between CIS and invasive carcinoma. The relationships between histologic features of CIS (cell size, nuclear pleomorphism, necrosis, extent of CIS) the patient's age, and the ER status of CIS also were evaluated. ER expression in pure CIS was compared with that of CIS with adjacent invasive carcinoma. Significant differences were observed between comedo CIS, which was frequently ER negative, and non-comedo and lobular CIS, which usually were positive. A predominance of large cells, independently from other features, was the best morphologic predictor of ER negative status in CIS. ER in CIS correlated with ER in invasive carcinoma in 98% of cases (r2 = 0.677). CIS without invasive carcinoma was more frequently ER weak or negative than CIS associated with invasive carcinoma. There was no difference in the overall percentage of nuclear staining in CIS of women of premenopausal versus postmenopausal age; however, a higher proportion of strongly positive cells occurred in the postmenopausal group. The authors conclude that the ER expression in CIS correlates with pathologic features of differentiation and is similar to that of invasive carcinoma.