Benzidine is a known human carcinogen and till 1990s was used as an intermediate in the synthesis of azo dyes. Formation of adducts with DNA, proteins and hemoglobin is an important marker of exposure to genotoxic chemicals and it may be an early indicator of development of cancer. In collaboration with NCI/NIH and University of Cincinnati Medical Centre, USA., four DNA adducts in exfoliated urothelial cells have been identified in the workers exposed to benzidine and benzidine based dyes. Of these, the predominant adduct which co-chromatographed with a synthetic N-(3¢-phosphodeoxyguanosin-8-yl)-N¢-acetylbenzidine standard and was the only adduct that was significantly associated with total benzidine urinary metabolites (r=0.68, p<0.0001). To the best of our knowledge, this is the first report to show that benzidine forms adducts in urothelial cells of the exposed humans and the predominant adduct formed is N¢-acetylated. For the analysis of excretion pattern of benzidine and its acetylated metabolites, an isotope dilution mass spectrometric technique using capillary GC/negative ion chemical ionization which possess a detection limit as low as 0.5, 0.8 and 1.5 ppt for benzidine, N-acetylbenzidine and N,N’-acetylbenzidine, respectively was developed. The findings demonstrated the usefulness of these non-invasive techniques for exposure/effect assessment in occupational settings.