Oxidative stress may be involved in the development of breast cancer. Manganese superoxide dismutase (MnSOD) is one of the primary enzymes that directly scavenge potential harmful oxidizing species. A valine (Val) to alanine (Ala) substitution at amino acid 16, occurring in the mitochondrial targeting sequence of the MnSOD gene, has been associated with an increase in breast cancer risk. We conducted a nested case-control study within the Nurses' Health Study cohort to examine the role of this polymorphism and its interaction with environmental factors with breast cancer risk. MnSOD genotype data are available from 968 incident cases of breast cancer diagnosed after blood collection in 1989 and 1990, but before June 1, 1998 and 1,205 matched controls. Compared with women homozygous for the Val allele, women homozygous for the Ala allele were not at an increased risk of breast cancer (multivariate odds ratio, 0.96; 95% confidence interval, 0.74-1.24). We did not observe any significant interactions between MnSOD genotype with alcohol consumption, postmenopausal hormone use, plasma antioxidant levels, or dietary sources of antioxidants. We did observe evidence that the MnSOD Ala allele may modify the relation of cigarette smoking with breast cancer risk. A nonsignificant increased risk of breast cancer among current smokers was limited to women homozygous for the Ala alleles compared with Val/Val never smokers (multivariate odds ratio, 1.41; 95% confidence interval 0.77-2.60; P for interaction = 0.03). These data suggest that the Ala allele of MnSOD may modify breast cancer risk among current smokers, but is not an independent risk factor for breast cancer.