Substantial quantities of isothiocyanates are released upon consumption of normal amounts of a number of cruciferous vegetables. Some of these naturally occurring isothiocyanates such as phenethyl isothiocyanate (PEITC), benzyl isothiocyanate (BITC) and sulforaphane are effective inhibitors of cancer induction in rodents treated with carcinogens. A large amount of data demonstrate that isothiocyanates act as cancer chemopreventive agents by favorably modifying carcinogen metabolism via inhibition of Phase 1 enzymes and/or induction of Phase 2 enzymes. These effects are quite specific, depending on the structure of the isothiocyanate and carcinogen. One of the most thoroughly studied examples of isothiocyanate inhibition of rodent carcinogenesis is inhibition of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung tumorigenesis by PEITC. This occurs because PEITC blocks the metabolic activation of NNK, resulting in increased urinary excretion of detoxified metabolites. Similar effects on NNK metabolism have been observed in smokers who consumed watercress, a source of PEITC. On the basis of these observations and knowledge of the carcinogenic constituents of cigarette smoke, a strategy for chemoprevention of lung cancer can be developed.