The identification of intragenic rearrangements is important for a comprehensive understanding of mutations that occur in some clinically important genes. Single nucleotide polymorphism haplotypes obtained from clinical sequence data have been used to identify patients at high risk for rearrangement mutations. Application of this method identified a novel 26-kb deletion of BRCA1 exons 14 through 20 in patients from multiple families with hereditary breast and ovarian cancer. Clinical sequence data from 5911 anonymous patients were screened for genotypes that were inconsistent with known pairs of canonical haplotypes in BRCA1 that could be explained by hemizygous deletions involving exon 16. Long-range polymerase chain reaction demonstrated that two of six samples identified by this search contained a deletion in the expected region encompassing exons 14 through 20. The breakpoint was fully characterized by DNA sequencing and demonstrated that the deletion resulted from Alu-mediated recombination. This mutation was also identified twice in a set of 982 anonymous specimens that had negative clinical test results, but uninformative haplotypes. Three additional occurrences of this mutation were found by testing 10 other patients with the indicative genotype. An assay for this mutation was added to a comprehensive clinical breast/ovarian cancer test and eight more instances were found in 20,649 probands. This multiexon deletion has therefore been detected in 15 different North American families with hereditary breast/ovarian cancer. In conclusion, this primarily computational approach is highly effective and identifies specimens using existing data that are enriched for deletion mutations.