Between 1968 and 1993, 43 cases of ductal carcinoma in situ of the breast (DCIS) were treated. We examined the extent of the cancer using multiple sections of surgical specimens and considered whether or not breast conserving treatment (BCT) is feasible for treatment of DCIS. In nine out of 40 patients (23%), extent of the cancer was classified as grade III, defined as extension over a 2.5-cm margin around the tumor. Among those nine cases, 67% (6/9) had extensive microcalcification on mammography. With respect to the correlation between the tumor size and the extent of the cancer, 29%(5/17) of the cases with a small size tumor, ie, 1.0 cm or less, showed grade III or greater. All tumors with a size of 1.1-2.0 cm showed grade II or lower, and 50%(4/8) of cases with a tumor size of 2.1 cm or more showed grade III or greater. Concerning the relationship between the histological subtype and the extent of the cancer, 22%(2/9) of comedo carcinomas and 23%(7/30) of noncomedo carcinomas showed grade III or greater. The histological extent thus showed no difference between comedo carcinoma and noncomedo carcinoma. It follows that, when BCT performed on DCIS consists of lumpectomy alone, BCT is feasible when the tumor size is 1.1-2.0 cm. without extensive microcalcification on mammography. However, BCT for comedo carcinoma should be approached with caution because of its malignant behavior, although there was no difference in histological extent between comedo and noncomedo carcinoma.