BRCA1 and BRCA2 germline mutations, mainly point mutations and other small alterations, are responsible for most hereditary cases of breast-ovarian cancer. However, the observed frequency of BRCA1 alterations is lower than that predicted by linkage analysis. Several large BRCA1 rearrangements have been identified with a variety of technical approaches in some families. We have developed a gene dosage assay based on real-time quantitative PCR and used it to extensively analyze 91 French families of breast-ovarian cancer in which no BRCA1 or BRCA2 point mutations was identified. This gene dosage method calculates the copy number of each BRCA1 exon to readily detect one, two, and three or more copies of BRCA1 target exons. In the series of 91 families at high risk of carrying BRCA1 mutations, we detected seven large rearrangements of the BRCA1 gene by using this real-time PCR approach. This simple, rapid, and semiautomated real-time quantitative polymerase chain reaction (PCR) assay is a promising alternative technique to Southern blot, bar code analysis on combed DNA, quantitative multiplex PCR of short fluorescent fragments, and cDNA length analysis for the detection of large rearrangements. Therefore, this technique should be considered as a powerful diagnostic method for breast/ovarian cancer susceptibility in clinical and research genetic surveys.