The extensive intraductal component (EIC) of primary breast carcinoma is a special spread pattern observed in the breast. Extensive intraductal component may extend diffusely over the entire breast. Therefore, EIC is considered to be an important risk factor for local recurrence in breast-conserving therapy. However, the pathogenesis of EIC remains uncertain. Whether or not the estrogen receptor (ER) has an influence on its biologic behavior has not been fully studied. A consecutive series of 142 breast carcinomas submitted to the pathology department were examined on step gross sections of 5.0 mm thick. Extensive intraductal component was determined and divided into three types. Estrogen receptor was examined using both immunohistochemistry (ER-IHC) and enzyme immunoassay (ER-EIA). Extensive intraductal component was found in 78 of 138 (56.52%) invasive carcinomas including invasive ductal carcinoma with a predominant intraductal component. Estrogen receptor-IHC positivity was 42.96% (61/142) in the invasive breast carcinoma. Estrogen receptor positivity showed no significant difference between EIC-positive and -negative cases, as well as between EIC and invasive main tumor in the EIC-positive cases. But within the EIC-positive group, ER positivity was found to be higher in the peripheral type of EIC-II and EIC-III than in the central type of EIC-I (P < 0.05). Although ER may not play an essential role in the pathogenesis of EIC, it has shown some significance in the development of peripheral type EIC because of its higher presence in the peripheral type of EIC-II and EIC-III than in the central type of EIC-I.