The growth properties of fibroblasts from the thoracic skin of patients with mammary cancers were compared to those of fibroblastic cultures from patients with benign lesions or having undergone surgery for non-neoplastic diseases. As expected, an inverse correlation was found between the doubling potential of fibroblasts in vitro and the donor's age for cells from patients with benign lesions; however no correlation, was found with cultures from cancer patients. Moreover, the latter group responded in an abnormal way to three biological parameters: anchorage dependence, colony formation on monolayers of normal human epithelial cells and saturation densities in overcrowded culture conditions. Skin fibroblasts from one patient with a benign lesion, whose mother had developed a breast cancer, displayed all the abnormal growth properties. Periodic controls of this patient resulted in the early detection of a carcinoma 3 years after the first operation for a benign microcystic lesion. Finally, we found that multiple subcultivations in overcrowded culture conditions cause the selection of a fibroblastic cell subset with greater growth potential which, in the cell strain tested, could invade foreign tissue in vitro.