Duct carcinoma in situ is now being detected with a frequency and at a size unknown prior to mammography. The majority of currently detected lesions are of limited extent and not associated with either occult invasion or axillary metastasis. For such limited duct carcinoma in situ, attempts at adequate local excision appear appropriate. Duct carcinoma in situ represents a number of biologically different processes that exhibit different frequencies of occult invasion and different risks for local recurrence after attempts at excision biopsy. The risks of local recurrence after a breast-conserving procedure without irradiation observing the selection criteria we employ can be estimated on the basis of the histologic subtype of the in situ carcinoma, the extent of disease, and the adequacy of the resection margins. In our prospective series, these risks ranged from 0 to 25 per cent for specific histologic subtypes at a median of 68 months of follow-up, with an overall frequency of recurrence of 12.6 per cent. All recurrences were local in the breast. Half were noninvasive disease, and all of the latter were initially treated by re-excision only. Other investigators report a similar experience. Invasive recurrences have been of minimal size, and all but one was free of nodal metastases. All patients are well at present. Three deaths have occurred secondary to cardiac disease.