In a review of 3040 cases of carcinoma of the breast of all types in the files of the Laboratory of Surgical Pathology at Columbia for the years 1960 to 1980, 267 cases were found in which the lobular neoplasia lesion coexisted with one of the usual forms of breast carcinoma. These patients had a separate and distinct, and of course malignant, clinicopathologic entity which is distinguished from benign lobular neoplasia occurring alone. Comparing these findings in lobular neoplasia coexisting with one of the usual forms of carcinoma with our findings in lobular neoplasia occurring alone, it was found that the patients with the latter lesion were younger. Three of the nine microscopic features studied in both forms of lobular proliferation were considerably more frequent in lobular neoplasia coexisting with carcinoma: (1) loss of cohesion of the cells filling up the lobules; (2) macroacini; and (3) a maximal amount of lobular neoplasia. The great majority of the forms of carcinoma that were found coexisting with lobular, neoplasia were well differentiated, small cell, intraductal, and tubular, and metastasized less often than carcinomas usually do. Carcinoma developed in the second breast three times more frequently in patients with lobular neoplasia preceding or coexisting with unilateral carcinoma than it did in patients without lobular neoplasia.