The profile of antioxidants in biological fluids and tissues may be helpful in assessing oxidative stress in humans. Plasma antioxidants can be decreased as compared to established normal values, in abnormal or subnormal conditions, for instance as a consequence of disease-related free radical production. Alternatively, plasma antioxidants may be below the normal range due to insufficient dietary supply. Therefore, the profile of antioxidants can be of use only in conjunction with other parameters of the oxidative stress status. This article examines the profiles of plasma antioxidants in oxidative stress-related conditions, e.g., diabetes and some other diseases, as well as smoking and smoking cessation.