Twenty-nine cases of invasive lobular carcinoma were analyzed, based on three aspects of the histology: 1) cellular features such as a monotonous proliferation of uniform small cells, 2) a single file or targetoid arrangement, and 3) loss of cell cohesion or dissociation of tumor cells. Twenty-four tumors which fulfilled these three criteria were appraised as cases of conventional lobular carcinoma, in a classic sense, while five others were a variant of this tumor. Individual tumor cells of lobular carcinoma were estimated to be well differentiated, both morphologically and functionally, revealing well developed intracytoplasmic organelles and a high percentage of alpha-lactalbumin content in the cytoplasm. Nevertheless, the tumor itself was characterized by a lack of any particular structural differentiation in the arrangement of cells. Based on the observation of the histologic features, invasive lobular carcinoma was subclassified into three groups, in situ predominant, intermediate, and diffuse infiltrating and with a definite correlation to the age of the patient and to the prognosis. Validity of this classification indicates that lobular carcinoma progresses gradually, even in the invasive phase, and can be categorized as a slowly growing subset of mammary carcinoma.