We conducted a nested case control study of 178 incident breast cancer cases and 356 controls in the Atherosclerosis Risk in Communities study. We evaluated the association between breast cancer and Val432Leu polymorphism in the CYP1B1 gene and the tetranucleotide repeats in intron 4 of the CYP19 gene. After adjustment for height, age at menopause, age at menarche, BMI, HRT, and alcohol intake, carriers of the Val/Leu or Val/Val genotype had a 1.45 fold (95% CI 0.85-2.47) greater odds of breast cancer than Leu/Leu carriers. There was no association of the breast cancer with any individual CYP19 allele. Compared to individuals homozygous with the 167 allele, odds ratios were close to 1.0 for the 167 heterozygous genotype and for the remaining tetranucleotide repeats combined. Our data shows no association between breast cancer and the Leu432Val polymorphism of the CYP1B1 gene or the tetranucleotide repeats of the CYP19 gene.