Fifty-two mammary carcinomas, 2 cm or less in diameter, were examined in order to clarify the morphology and biology of microinvasion. The morphological characteristics of microinvasion of carcinomas include: (i) a loss of myoepithelial cells and a rupture with concomitant loss of collagen IV and laminin in the basement membrane of involved mammary glands; and (ii) budding of carcinomas from the rupture into the stroma. When microinvasion was defined as a rupture of < 200 μm in the basement membrane with invasion, the number of microinvasions per 1 mm of basement membrane was larger in the tumors in which the area of invasion was larger. The prevalence of microinvasion showed a significant correlation with lymph node metastasis and the rate of histological deviation, while no correlation of expression of either estrogen receptors or progesterone receptors and c-erbB-2 protein was found. The study clarified that the early invasion of mammary carcinomas could be detected by the immunohistochemical method using anti-smooth muscle actin, laminin and collagen IV antibodies. The study also suggested that microinvasion might be an indicator of lymph node metastasis in mammary carcinomas ≤ 2 cm diameter.