Treatment with statins are known to lower plasma and low-density lipoprotein (LDL) cholesterol levels with resultant prevention and regression of atherosclerosis. It has been recently suggested that the action of the statins may also have a direct effect on other mechanisms involved in the atherosclerotic plaque formation. Thus, we investigated whether simvastatin could have an antioxidant effect on plasma lipoproteins. The rate of oxidation of LDL and high-density lipoproteins (HDL) was measured by conjugated diene formation with and without the addition of increasing concentrations of simvastatin (in vitro) and in patients with and without treatment with simvastatin (in vivo). A strong correlation was observed between increasing simvastatin concentration and the lag phase, a negative correlation was observed for maximal rate and maximum diene production in LDL samples (r2 = +0.97, p <0.0001; r2 = -0.92, p <0.0001; r2 = -0.98, p <0.0001, respectively). For HDL no clear correlation could be established with the lag phase, but a strong negative correlation was also observed between simvastatin concentration and maximal rate and maximum diene production (r2 = -0.69, p <0.01; r2 = -0.98, p <0.0001, respectively). After 6 hours of oxidation the production of aldehydes in LDL and HDL was lower (30% and 5%, respectively) in samples obtained during simvastatin therapy with respect to those obtained without treatment. The 2,4-decadienal showed a decrease of 37% and 64% (p <0.05) in both oxidized-LDL and oxidized-HDL particles, respectively, with simvastatin treatment. Our findings demonstrate that simvastatin acts as an antioxidant in lipoprotein particles and, together with its lipid-lowering properties, could play an important role in preventing atherosclerosis.