We screened a population-based series of breast cancer cases diagnosed before the age of 55 for mutations in BRCA1 and BRCA2. Pedigree information from the mutation carriers was used to estimate penetrance and the proportion of familial risk of breast cancer due to BRCA1 and BRCA2. We identified eight (0.7%)BRCA1 and 16 (1.3%)BRCA2 mutation carriers in 1220 breast cancer cases (actual sample size 1435 adjusted for 15% polymerase chain reaction failure rate). This evidence concerns the gene BRCA1 and breast cancer.