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Impairment in oxidative stress/antioxidant balance is an important trigger for a variety of diseases. As an antioxidant molecule on high-density lipoprotein (HDL), paraoxonase (PON1) contributes to the antioxidant mechanisms by removing oxidised lipids both on HDL and low-density lipoprotein (LDL). In this chapter, we will document and evaluate the results of our studies on healthy, atheroscleoric and diabetic cases which showed that (a) PON1, superoxide dismutase (SOD) and arylesterase probably work in a collaboration against oxidative stress, especially superoxide radical scavenging; (b) PON1 and SOD activities concomitantly decrease with the oxidative stress & severity of disease (higher HbA1c values in diabetics, more diseased vessels in atherosclerosis) while catalase (CAT) acts the opposite way; (c) Since PON1 activity and erythrocte thiobarbituric acid reactive substances (eTBARS) levels are affected by traditional risk factors (hypertension, aging and gender), determination of arylesterase activity might be a better indicator of antioxidant activity of PON1; (d) SOD activity has the greatest variability in regard to PON1 phenotype therefore it's important to define the PON1 polymorphism as well as PON1, arylesterase and other antioxidant enzyme activities [ABSTRACT FROM AUTHOR]