Tobacco smoke constituents, 4-aminobiphenyl (4-ABP) and polycyclic aromatic hydrocarbons (PAH) possess carcinogenic properties as their reactive metabolites form DNA adducts. We studied the formation of 4-ABP- and PAH-DNA adducts in induced sputum, a non-invasively obtainable matrix from the lower respiratory tract, of smokers (n=20) and non-smokers (n=24) utilizing a semi-quantitative immunohistochemical peroxidase assay. Smokers had significantly higher levels of 4-ABP-DNA adducts as compared to non-smokers (0. 08+/-0.02 versus 0.04+/-0.01, P=0.001, density of immunohistochemical staining), and the levels of adducts were related to current smoking indices (cigarettes/day: r=0.3, P=0.04 and tar/day: r=0.4, P=0.02). Likewise, smokers had elevated levels of PAH-DNA adducts as compared to non-smokers, however, the differences was not statistically significant (0.13+/-0.02 versus 0. 08+/-0.02, P=0.07). The levels of PAH-DNA adducts were only significantly related to the amount of tar consumed per day (r=0.4, P=0.04) but not to the number of cigarettes smoked per day. Neither the levels of 4-ABP-DNA adducts nor those of PAH-DNA adducts were related to smoking history index (pack years). Further, the levels of 4-ABP-DNA adducts were correlated with those of PAH-DNA adducts (r=0.4, P=0.02). We conclude that immunohistochemistry of 4-ABP-DNA adducts in induced sputum is a specific approach to assess current exposure to tobacco smoke in the lower respiratory tract, however, in the case of PAH-DNA adducts, such analysis is less specific as it does not explicitly reflect the magnitude of the exposure.