Two small series (nine cases each) of human female breasts were collected to compare the morphological changes of mammary glandular trees contralateral to primary breast cancer and those collateral to symptomatic benign lump. Each whole mammary gland was analysed by a submacroscopic scrutiny method using a stereomicroscope. Interesting and suspicious samples were removed for routine histology. Benign subclinical lesions were indifferently present in both series: Spheric cysts (5:5), sclerosing adenosis (3:3), intraductal papillomas (1:1), fibroadenomas (3:1). On the contrary proliferative epithelial lobular lesions with various degree of atypia i.e. atypical lobules (Grades IV-V according to Wellings), were detected only in the first series (p < 0.01). These data agree completely with the hypothesis of a systemic nature of breast cancer and support indirectly the possible predictive value of atypical lobules in bioptic specimens for the subsequent development of cancer in collateral and/or contralateral breasts.