We monitored serum malondialdehyde (MDA), an indicator of lipid peroxidation, to determine whether active cigarette smoking causes oxidative damage. The results revealed that the concentration of serum MDA was higher in cigarette smokers than in nonsmoking control subjects. No relation was found between lipid peroxidation and the number of cigarettes smoked by an individual. This finding supports the hypothesis that oxidative damage in smokers is due to the number of hours of active exposure to cigarette smoke.