Within mitochondria, manganese superoxide dismutase (MnSOD) provides a major defence against oxidative damage by reactive oxygen species (ROS). An alanine-9valine (Ala-9Val) polymorphism in the mitochondrial targeting sequence of MnSOD has been described and has recently been associated with risk of human breast cancer. Our present case-control study was performed to explore the association between MnSOD genetic polymorphism and individual susceptibility to breast cancer. Ala-9Val polymorphism in the signal sequence of the protein for MnSOD was determined using the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay in a study population. There was no significant difference in risk for breast cancer development between patients positive and negative for the MnSOD Ala allele with adjusted odds ratio (OR): 0.86 (95% confidence interval (CI) 0.43 to 1.72). When MnSOD Ala was combined with either cytochrome P450 1B1 CYP1B1*1 and catechol O-methyltransferase COMT-L (V158M) genotypes, the risk for developing breast cancer was significantly increased in patients with a body mass index (BMI) greater than 24 kg m(-2) (OR: 1.42 (95%CI = 1.04-1.93)). Copyright (C) 2004 John Wiley Sons, Ltd.