Breast development, one of the first signs of puberty, is closely associated with age at menarche; and early menarche is in turn a well-established risk factor for female breast cancer. We examined the relationships between the onset of puberty and gene variants for certain enzymes that regulate hormone metabolism among 137 healthy nine-year-old girls from two pediatric clinics. High-activity CYP17 alleles, involved in estrogen formation, and high-activity CYP1A2 and CYP1B1 alleles, whose gene products metabolize estradiol, were not associated with pubertal stage. High activity CYP3A4, but not CYP3A5, which primarily metabolizes testosterone, showed a striking association with the onset of puberty (adjusted odds ratio, 3.21; 95% confidence interval, 1.62-6.89 for the genotype 0-1-2 rapid alleles). Of the homozygous CYP3A4*1B/1B girls, 90% had reached puberty; whereas, for the low-activity homozygous CYP3A4*1A/1A individuals, only 40% had done so. In heterozygotes, 56% had reached puberty. CYP1B1, CYP3A4, and CYP3A5 rapid variants were more common in African-American than in Hispanic or Caucasian girls.