The role of K(+)-Cl(-)-cotransport in apoptosis in human cancer cells was investigated. N-Ethylmaleimide, a K(+)-Cl(-)-cotransport activator, induced apoptosis in a dose-dependent manner in HepG2 human hepatoblastoma cells. N-Ethylmaleimide induced Cl(-)-dependent K(+) efflux, indicating that K(+)-Cl(-)-cotransport is functionally present in HepG2 cells. Calyculin-A and genistein, inhibitors of K(+)-Cl(-)-cotransport, significantly prevented both K(+)-Cl(-)-cotransport activation and apoptosis induced by N-ethylmaleimide. These results demonstrate, for the first time, a novel role for K(+)-Cl(-)-cotransport in apoptosis in human hepatoma cells. These results further suggest that K(+)-Cl(-)-cotransport may be a valuable target for therapeutic interventions for human hepatoma.