Breast cancer multifocality was studied in mastectomy specimens by correlated specimen radiography and histologic techniques. The patients chosen for study were comparable to those eligible for breast-conserving surgical therapy. Two study groups, one with 282 invasive cancers (Tl-2) and the other with 32 intraductal cancers, were selected from a group of 399 consecutive cases by omitting patients who were clearly, or very probably, not candidates for breast-conserving surgical therapy according to current trial criteria. Omitted patients included those with clinically and/or radiologically multifocal cancers and patients with tumor extension into the chest wall or skin (7%). Also excluded were the so-called diffuse invasive cancers @%), the clinically and radiologically occult tumors (3%), and the invasive cancers larger than 5 cm (3%). Of the 282 invasive cancers, 105 (37%) showed no tumor foci in the mastectomy specimen around the reference mass. In 56 (20%) tumor foci were present within 2 cm, and in 121 (43%) tumor was found more than 2 cm from the reference tumor. In 75 (27%) the tumor foci beyond 2 cm were histologically noninvasive cancers, and in 46 cases (16%) they contained invasive cancers as well. A comparison between the group with reference tumors less than 2 cm and the group with reference tumors more than 2 cm in size showed no significant difference between the groups in terms of presence or absence of tumor foci or distance of tumor foci from the reference tumor. If the 264 invasive cancers in this series that were 4 cm or less in diameter had been removed with a margin of 3 to 4 cm, 7% to 9% of the patients would have had invasive cancer left in the remaining breast tissue, and 4% to 9% would have had foci of noninvasive cancer left in the remaining breast tissue. On the basis of the data on the distribution of tumor at different distances from the reference tumor, the current study estimates the expected rates of local recurrences after breast-conserving surgical procedures relative to the extensiveness of the excision. The possible impact of postoperative local radiation therapy on the rates of expected local recurrence is discussed. Cancer 56:979-990, 1985. EPORTED HIGH rates of multifocality of breast cancer R approximately 40%-60%, based on pathologic studies, have been for many years a major deterrent to attempts to treat breast cancer with less mutilating surgical procedures than radical or total mastectomy. Nevertheless, a few nonrandomized studies were camed out that indicated that, in selected groups of patients, the extensiveness of the local surgical therapy does not