An increased level of serum estrogen is one marker of breast cancer risk. We have recently reported that increased risk of advanced breast cancer is associated with a common allele of the cytochrome P450c17alpha gene (CYP17), designated A2. We now show that CYP17 genotype is associated with serum hormone levels among 83 young, nulliparous women. Serum estradiol (E2) levels measured around day 11 of the menstrual cycle were 11 and 57% higher (P = 0.04), respectively, among women hetero- and homozygous for the CYP17 A2 allele compared to A1/A1 women. Similarly, around cycle day 22, E2 levels were 7 and 28% higher (P = 0.06), and progesterone levels were 24 and 30% higher (P = 0.04), respectively. These data provide direct evidence of genetic control of serum hormone levels.