Increased oxidative stress has been speculated to be one possible mechanism of ethanol toxicity. This study evaluates malondialdehyde (MDA) and protein carbonyl content in serum as markers of oxidative stress and DNA damage in lymphocytes in the same patients with chronic alcoholism. Patients with chronic alcoholism showed a significant increase in MDA levels and protein carbonyl content of their serum as compared with non-alcoholic control subjects. Increases in endogenous and H2O2-induced DNA damage were also observed in lymphocytes of patients with chronic alcoholism. In addition, there were significant correlations between endogenous and H2O2-induced DNA damage and serum MDA or protein carbonyl content in patients with chronic alcoholism. These results clearly indicate the presence of oxidative stress in patients with chronic alcoholism.