The growth properties of thoracic skin fibroblasts derived from patients with breast cancers were compared to those of fibroblastic cultures derived from patients with benign lesions or operated for nonneoplastic reasons. The parameters we measured included comparison between growth fraction and cell density, growth in overcrowded culture conditions, growth in low serum concentration, anchorage dependence, and lifespan. We found that the lifespan decreases significantly as a function of donor's age for cells of patients with benign lesions, but not for cancer patients. Furthermore, fibroblastic cultures from patients with mammary carcinomas responded in an abnormal way to all the biological parameters for which they were tested. Skin fibroblasts from patients with benign lesions or from "normal donors," with one exception, showed some but not all these abnormal properties. The patient with a benign lesion, whose cells behaved as those from cancer patients, developed a breast cancer within 3 yr.