Malondialdehyde (MDA) is a major lipid peroxidation product that is mutagenic and tumorigenic. MDA-modified DNA adducts have been detected in animal and human tissues and may be a marker of human cancer risk. An immunohistochemical method, using a previously generated monoclonal antibody specific for MDA-DNA adducts, has been developed for the detection and quantification of DNA damage in human oral mucosa cells. The method was used initially on woodchuck liver cells treated with and without MDA, and then applied to the detection of adducts in oral mucosa cells of smokers and non-smokers. Levels of DNA damage were elevated in 25 smokers (mean relative staining intensity 97 +/- 41) compared with 25 age-, race- and sex-matched non-smokers (74 +/- 17, P < 0.02). These results demonstrate that MDA-DNA adducts can be measured in single cells of human samples by an immunohistochemical method. This methodology provides a simple way to monitor MDA-DNA damage and should be useful for studies investigating the role of exogenous and endogenous agents in oxidative stress and carcinogenesis.