Lipid peroxidation of important components of cellular membranes such as polyunsaturated fatty acids is caused by free oxygen radicals and is considered to be one of the main causes of hepatocyte injury. In order to investigate whether lipid peroxidation increases in acute viral hepatitis, plasma malondialdehyde (MDA) concentration was determined in 27 healthy subjects and in 36 patients suffering acute viral hepatitis. Plasma MDA levels were determined by fluorometric measurement of fluorescense intensity of MDA product and thiobarbituric acid reaction. Mean plasma MDA concentration was 0.822±0.533 μmol/L for the acute hepatitis A group (n=11), 1.836±0.515 μmol/L for the acute hepatitis B group (n=25), and 0.589±0.227 μmol/L for the control group. Plasma MDA levels of patients with acute hepatitis B were found to be higher than those in the control group and these differences were significant. (p<0.001). It is concluded that the increased MDA levels found in acute hepatitis B and C prove the increase in lipid peroxidation.