The herbal drug aristolochic acid (AA), which is derived from the Aristolochia species, has been associated with the development of a novel nephropathy, designated as aristolochic acid nephropathy (AAN), and with human urothelial cancer. The major components of the plant extract AA are nitrophenanthrene carboxylic acids, which, after metabolic activation, are genotoxic mutagens. The major activation pathway of AA involves reduction of the nitro group, primarily catalyzed by NAD(P)H:quinone oxidoreductase (NQO1), to an electrophilic cyclic N-acylnitrenium ion that reacts preferentially with purine bases to form covalent DNA adducts. These specific AA-DNA adducts have been identified and detected in experimental animals exposed to AA or botanical products containing AA, and in urothelial tissues from AAN patients. In rodent tumors induced by AA the predominantly formed DNA adduct 7-(deoxyadenosin-N6-yl)aristolactam I has been associated with the activation of ras oncogenes through the characteristic transversion mutation AT-->TA. This mutation has been identified in the p53 gene of urothelial tumors of a patient with AAN (induced by use of a herbal product) and in several patients suffering from Balkan endemic nephropathy (BEN) with specific AA-DNA adducts. This is a rare example of a human cancer causally linked to a distinct environmental exposure (ie, use of a herbal product), where the carcinogenic process of initiation is well established, linking formation of carcinogen-specific exposure (specific DNA adduct formation) with the presence of characteristic human tumor mutations.