Abstract Oxidative stress may enhance prostatic carcinogenesis. A polymorphism [valine (V) → alanine (A)] of manganese superoxide dismutase (MnSOD), the primary antioxidant enzyme in mitochondria, has been recently associated with prostate cancer. We examined the relationship between prostate cancer and the MnSOD polymorphism and its interactions with baseline plasma antioxidant levels (selenium, lycopene, and α-tocopherol) and β-carotene treatment among 567 cases and 764 controls nested in the prospective Physicians' Health Study. We found little overall association between MnSOD polymorphism and prostate cancer risk; however, this polymorphism significantly modified risk of prostate cancer associated with prediagnostic plasma antioxidants (Pinteraction ≥ 0.05). Among men with the AA genotype, high selenium level (4th versus 1st quartile) was associated with a relative risk (RR) of 0.3 [95% confidence interval (CI), 0.2-0.7] for total prostate cancer; for clinically aggressive prostate cancer, the RR was 0.2 (95% CI, 0.1-0.5). In contrast, among men with the VV/VA genotype, the RRs were 0.6 (0.4-1.0) and 0.7 (0.4-1.2) for total and clinically aggressive prostate cancer. These patterns were similar for lycopene and α-tocopherol and were particularly strong when these antioxidants and selenium were combined; men with the AA genotype had a 10-fold gradient in risk for aggressive prostate cancer across quartiles of antioxidant status. Men with AA genotype who were randomly assigned to β-carotene treatment (versus placebo) had a RR of 0.6 (95% CI, 0.2-0.9; Pinteraction = 0.03) for fatal prostate cancer, but no significant association was observed in men with the VV/VA genotype. Both endogenous and exogenous antioxidants play an important and interdependent role in preventing clinically significant prostate cancer.