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Background. The authors have reviewed their institution's experience with conservative surgery and radiation therapy for early stage breast cancer with the goal of defining the impact of infiltrating lobular histology (ILC) on the local recurrence rate. Also, they have examined the preoperative mammograms of the ILC patients to determine if mammographic features could be used to predict treatment outcome. Methods. Between January 1, 1980, and December 31, 1987, 402 cases of Stages I and II breast cancer were treated with conservative surgery and radiation therapy (BCT) at William Beaumont Hospital. Each patient had at least an excisional biopsy. Radiation consisted of 45-50 Gy to the entire breast followed by a supplemental boost dose, so that a minimum of 60 Gy was delivered to the tumor bed. Thirty cancers were classified histologically as infiltrating lobular carcinoma (ILC), 346 as infiltrating ductal carcinoma (IDC), and 26 as other. Median follow-up is 60 months. Results. There was no significant difference in 5 year actuarial local recurrence rates between ILC and IDC, 3.3 versus 4.2%, respectively, (P = not significant). Preoperative mammograms were retrospectively reviewed for 29 of the 30 ILC patients. A spiculated opacity was the most common primary mammographic finding (63%), followed by architectural distortion (17%), poorly defined opacity (7%), and negative (7%). Of the patients who had a preoperative primary mammographic finding of a spiculated opacity, 55% underwent reexcision after the initial excisional biopsy, and residual invasive carcinoma was found in 18% of the reexcision specimens. In contrast, of the patients with a primary mammographic finding of an architectural distortion, poorly defined opacity, or negative, 89% underwent reexcision after an initial excisional biopsy, and residual invasive carcinoma was found in 100% of the reexcision specimens. Conclusions. Infiltrating lobular carcinoma does not have a worse local recurrence rate compared with IDC when each is treated with breast-conserving therapy. The primary finding on preoperative mammograms in patients with ILC may prove to be a useful tool for predicting the likelihood of residual carcinoma in the breast after initial excisional biopsy. Cancer 1994; 74:640-7.