We prospectively screened a hospital-based population of 1000 successive breast cancer patients receiving adjuvant radiotherapy for predictive factors associated with the presence of BRCA1 and BRCA2 mutations. We offered genetic counseling and DNA analysis to selected patients. About 52% of patients showed at least one presumed predictive factor. Hundred and thirty-seven patients underwent DNA analysis. We identified 14 deleterious mutations (10.2%, 95% CI: 5.2-15.3%): 8 BRCA1 mutations and 6 BRCA2 mutations and 14 variants of uncertain clinical significance. Ovarian cancer in the family history was the only factor significantly associated with the presence of a disease-causing mutation (P < 0.01). Eight of the 14 (57%) mutation carriers had no affected first-degree relatives and in 4 of these there was no family history of breast or ovarian cancer. Clinicians should offer genetic counseling and DNA testing to breast cancer patients from families with breast and ovarian cancer, and to patients who are younger than 45 years when they are diagnosed with breast cancer.