A review of 205 consecutive breast carcinomas of ductal type disclosed that 45 concomitantly involved the terminal ducts of the lobule. Carcinoma presumably invaded the lobule by intraluminal extension from lactiferous ducts. The presence of this type of lobular involvement may be mistaken for lobular carcinoma in situ and is especially confusing when found in the breast remote from the grossly detectable ductal neoplasm. The diagnosis of lobular carcinoma has important therapeutic implications for the second breast because of the high frequency of bilaterality in that disease compared with ductal carcinoma. In our group of ductal carcinomas with lobular invasion, short term follow-up and examination of tissue from the opposite breast in 10 patients indicates that the frequency of bilaterality for lobular carcinoma is not attained. Therefore, it is important to sharply separate lobular carcinoma in situ from lobular invasion by ductal carcinoma. Carcinoma in a lobule is not necessarily lobular carcinoma.