Oxidative stress enhances carcinogenesis due to DNA damage. Manganese superoxide dismutase (MnSOD) Val16Ala polymorphism has been recently associated with breast and prostate cancer. The role of oxidative stress in male breast cancer is poorly investigated due to the low prevalence of this neoplasia. We studied the relationship between prostate cancer (PC), male (MBC) and female breast cancer (FBC) and this polymorphism in a case-control study. Human genetic polymorphism Val16Ala of MnSOD was obtained from blood and paraffin-embedded tumor samples. The polymorphism was determined in 11 cases of MBC, 51 cases of PC, 89 cases of FBC and 372 age-adjusted healthy controls by polymerase chain reaction-restriction fragment length polymorphism techniques using restriction enzyme Hae III. Chi-square or Fisher test were used to compare the MnSOD frequency distribution. The observed genotypic frequencies of all samples were AA = 9.6% (n = 50), VV = 25.4% (n = 133) and AV = 64% (n = 340), all at Hardy-Weinberg equilibrium. Breast and prostate cancer risk was elevated in male and female patients with the Ala/Ala genotype compared to controls (p = 0.006, odds ratio = 2.5, 95% confidence interval 1.393-4.541). Even though the frequency of the Ala allele was low (9.6%) in the studied population, these data support the hypothesis that MnSOD and oxidative stress play a significant role in breast cancer risk both in males and females and also brings new information on the role of this polymorphism in prostate cancer. This is the first study which provides some evidence that genetic polymorphism in the MnSOD gene may be associated with an increased risk of male breast cancer. Studies with a larger sample size are needed to confirm the findings.