Many isothiocyanates (ITCs), some of which are abundant in cruciferous vegetables, have been repeatedly shown to inhibit carcinogenesis in a variety of rodent organs. However, several naturally occurring ITCs also promoted bladder tumorigenesis in rodents, raising the question of whether ITCs behave differently in bladder cells. Alternatively, the observed carcinogenic effects of ITCs may result from prolonged exposure of the bladder epithelium, where the tumors originate, to high concentrations of electrophilic ITCs in the urine. Ingested ITCs are almost exclusively excreted and highly concentrated in the urine as N-acetylcysteine conjugates (NAC-ITC). While several NAC-ITCs also are known anticarcinogens, they are unstable and readily dissociate into parent ITCs. In this study, ITCs, including those that have carcinogenic potential in the rodent bladders, induced apoptosis and/or arrested cell-cycle progression in 2 human bladder carcinoma lines (UM-UC-3 and T24) at 7.5-30 micromol/L. Multiple caspases, including caspase-9, -8, and -3, as well as poly(ADP-ribose)polymerase, were cleaved upon ITC exposure. The ITCs blocked cell-cycle progression at the G(2)/M and/or S phases in these cells and downregulated several cell-cycle regulators. However, further increases in ITC concentrations abolished their activities, described above. These findings show that urinary ITC concentrations may need to be maintained at low micromolar concentrations for bladder cancer prevention.