• Fifty breasts with nonpalpable ductal carcinoma in situ (DCIS) were examined for the presence of microinvasion, multicentricity, and number of involved ducts to see if the biopsy specimen could have predicted the findings in the remainder of the breast. When DCIS was an incidental finding, fewer ducts were involved and no evidence of either microinvasion or multicentricity was found. Solid and cribriform DCIS were rarely multicentric or microinvasive; micropapillary DCIS was often multicentric, rarely microinvasive; comedocarcinoma was more likely to be both microinvasive and multicentric. Ductal carcinoma in situ as an incidental finding may be treated by excision alone; papillary and micropapillary DCIS are best treated by therapy aimed at the entire breast, although axillary dissection may not be required. Therapy for comedocarcinomas should include the entire breast and the axillary nodes. (Arch Surg 1989;124:29-32)