The antioxidant properties of vitamin C may be involved in the prevention of cancer. The correlation between dietary vitamin C intake as estimated by a dietary questionnaire and plasma ascorbic acid (AA) was examined in 68 nonsmoking male volunteers aged 30-59 years. Determinants of plasma AA as well as interrelationships between various antioxidants in plasma were also explored. The determinants of plasma AA were examined by a multiple regression model containing dietary vitamin C, calories, body weight, and amount of beverages consumed. Higher vitamin C intake (P < 0.0002) increased plasma AA, while greater body weight (P < 0.005) decreased plasma AA. A significant correlation (r = 0.43; P < 0.0003) between vitamin C intake and plasma AA was observed. There was a negative correlation between plasma AA and plasma uric acid (r = -0.32; P < 0.007) and positive associations between plasma beta-carotene and plasma alpha-tocopherol (r = 0.39; P < 0.001) and between plasma beta-carotene and plasma glutathione peroxidase (r = 0.32; P < 0.008). Vitamin C supplement users had higher plasma AA compared to nonusers. The relationship between plasma AA and vitamin C intake appears to be curvilinear with the non-supplement users at the linear part of the curve and the supplement users at the plateau. Plasma AA is an appropriate biomarker, in our subjects, of dietary vitamin C except for people consuming large amounts of this vitamin either in their diet or in supplemental form.