Fifty-one women (29 to 75 years of age) with 55 cancers (ductal carcinoma in situ [DCIS] or ductal carcinoma in situ with microinvasion [DCISM] were studied by comparing biopsy specimens with mastectomy specimens. Presentation, histologic type, nuclear grade, microscopic duct counts, multicentricity, and microinvasion were correlated. Forty-seven percent of the cancers (26 of 55) were detected by mammography, 18% (ten of 55) were incidental to benign disease, and 35% (19 of 55) were palpable or exhibited nipple abnormality. Incidental tumors were all DCIS, averaged seven ducts, and showed no residual tumor during mastectomy. Mammographic lesions averaged 117 ducts (31% [eight of 26] were DCISM and 42% [11 of 26] were multicentric). Most comedocarcinomas that showed a high incidence of microinvasion were in this group. Clinical lesions averaged 110 ducts (42% [eight of 19] were DCISM and 68% [13 of 19] were multicentric). Three had nodal metastases. Mammographic and clinical tumors in the quantitative range of the incidental group (50 ducts) showed significant differences from it for all variables studied. Histologic and quantitative study of these tumors is necessary to best guide treatment. Incidental tumors, however, may only need observation.