Seventy-nine patients with mammographically detected foci of duct carcinoma in situ (DCIS) of histologically confirmed extents of 25 mm or less, were treated by tylectomy without irradiation or axillary dissection. Adequacy of excision was confirmed histologically, by radiographic-pathologic correlation and by postoperative mammographic examination. Eight patients (10.1%) have recurred locally in the immediate vicinity of the biopsy site. Four patients developed recurrent in situ disease identified mammographically, and all were initially treated by reexcision. One of these patients subsequently elected to undergo mastectomy; no residual in situ or invasive disease was detected in the breast or in axillary lymph nodes. Four patients developed recurrent invasive disease; 50% of these recurrences were detected mammographically. All patients were treated by mastectomy with node dissection. Three had confirmed minimal invasive carcinomas and were N0, one patient had a 13-mm invasive lobular carcinoma with a single Group I micrometastasis. All patients, including those treated for a recurrence, are presently free of disease but three patients died of heart disease. Nuclear grade would appear to identify subsets of DCIS more likely to produce local failure after tylectomy alone. Duct carcinoma in situ with high-grade nuclear morphology and comedo-type necrosis was associated with a 19% local recurrence rate after an average interval of 26 months; only one of ten patients with intermediate-grade DCIS developed a local recurrence at 87 months; and none of 33 patients with DCIS of micropapillary/nonnecrotic cribriform type and low-grade nuclear morphology developed local recurrence in the follow-up period.