Hereditary breast carcinomas constitute about 10% of all malignant mammary tumors, but the selection criteria to identify a high-risk population carrying BRCA1 mutations are not yet well-defined. We have collected 51 pedigrees of familial breast cancer, 16 pedigrees of familial breast and ovarian cancer, and 30 cases of early-onset breast cancer (ฯฝ35 years of age) without any family history of breast cancer. The index cases of the 97 selected families were further subdivided into three groups based on histopathological parameters: group A (n ฯญ 19) was characterized by tumor grade III, negative estrogen and progesterone receptors, and high proliferative rate; group B (n ฯญ 20) was characterized by grade I-II tumors, positive hormonal receptors, and low proliferative rate; and group C (n ฯญ 58) was not homogeneous for the histopathological criteria. The aim of our study was to evaluate, in patients with a family history of breast cancer or with early diagnosis of breast cancer, the incidence of BRCA1 mutation on the basis of tumor phenotype. We found the highest rate of BRCA1 mutations in group A (53%), and low frequencies in groups B (5%) and C (0%). Our data strongly indicate that an aggressive tumor phenotype in patients with a positive family history or early diagnosis identifies a population with high probability of carrying BRCA1 mutations.