Exposure to sex hormones is a major risk factor for breast cancer and current treatments include hormone modifying drugs, among them aromatase inhibitors. We studied the association of CYP19 (Val(80) and [TTTA](n)) polymorphisms, the gene translated to aromatase, and the risk of breast cancer in BRCA carriers and noncarriers. The study consisted of 958 cancer cases and 931 healthy controls, including 474 carriers and 1,415 noncarriers. Cases and controls came from a population-based study of breast cancer in Israel, enriched with BRCA carriers from a clinical familial cancer service. Val(80) G/G genotype was associated with significantly increased risk of breast cancer compared with the Val(80) A/A genotype in BRCA1 carriers ages <50 years (odds ratio, 2.81; 95% confidence interval, 1.09-7.22; P = 0.032) but not in BRCA2 carriers or noncarriers of any age. A similar magnitude suggestive association, although nonstatistically significant, was found between Val(80) polymorphism and estrogen receptor-negative status of the breast tumors. A common haplotype composed of the Val(80) G allele and three haplotype-tagging single nucleotide polymorphisms (rs727479, rs10046, and rs4646) in the CYP19 coding region showed a trend to association with breast cancer risk in BRCA1 carriers ages <50 years. Published expression data show higher estrogen levels with higher repeats in [TTTA](n) found in linkage disequilibrium with Val(80). The present study suggests that the CYP19 Val(80) polymorphism and a haplotype that includes this polymorphism are associated with increased breast cancer risk in young women with BRCA1 mutations.