The heterozygous CYP1B1 (Val/Leu) genotype was associated with a significantly increased risk of premenopausal breast cancer (OR = 2.00, 95% 1.05–3.81; the risk associated with the homozygous genotype (Leu/Leu) did not reach significance (OR = 2.40, 95% CI 0.43–13.38). This evidence concerns the gene CYP1B1 and breast cancer.