The hormonal etiology of breast cancer is well-established. Many studies have assessed whether polymorphisms in steroid hormone metabolism genes are associated with breast cancer risk. We measured the CYP17A1 -34T4C (c.-34T4C) promoter polymorphism in a population-based study of 1,404 Australian women with breast cancer diagnosed before age 60 years (case probands), 1,903 relatives, and 788 controls. Within-family analyses suggested the CC genotype was associated with, on average, a small increased risk. This finding appeared to be influenced by the families of three early-onset case probands with multiple affected sisters. CYP17A1 mutation screening revealed a case proband diagnosed at age 38 years who had a germline proteintruncating mutation (c.775C4T, p.Arg239X), which results in a nonfunctional enzyme and has been reported in a male compound heterozygote with 17 a-hydroxylase deficiency. This mutation was carried by both sisters diagnosed with breast cancer at ages 34 and 42 years, but not by a 57-year-old unaffected sister. It was not found in any of the other tested case probands (48 with multiple-affected relatives and 241 randomly selected) or controls. This study suggests there may be rare mutations in steroid hormone metabolism genes associated with a high dominantlyinherited breast cancer risk, and demonstrates how ''high-risk susceptibility genes'' might be discovered using population-based case-control-family studies.